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WATER BALANCE IN THE ONION ROOT: RELATION OF VOLUMEINTAKE TO VOLUME EXUDATE OF EXCISED ROOTS

AND ISOLATED ROOT SEGMENTS

HILDA F. ROSENE

(WTITH TWO FIGURES)

IntroductionIn order to effectively study water balance and ion exchange in higher

plants it is important to obtain quantitative data concerning the mechanismof transport in root tissue per se. Such knowledge is also of significance incell dynamics since water balance is a property common to all cells.Although a careful study involving simultaneous determinaton of bothabsorption and exudation in a single isolated root or portion of root is essen-tial to a complete analysis of the dynamics of polar transport in higherplants, no such study has appeared heretofore. The present investigationwhich was confined to excised onion roots and isolated root segments shouldbe extended to include other types of roots including those which have beengrown by tissue culture technique.

Young onion roots (less than two weeks old) grown in water culture showpolar apical-basal differences in rates of water intake which are character-istic of root tissue since these differences are maintained both before andafter the roots have been isolated from the bulb and developing leaves;although water intake occurs at all levels, higher rates appear in the rela-tively more basal regions (16).

Unit volume of transport at a given axial level in an excised onion rootmay be just as large or larger than when it had the supposedly added forceof the bulb and developed leaves to reinforce or supplant its own force inthe transport of water in a saturated atmosphere. There is, of course, noconclusive evidence which shows whether or not the bulb and developingleaves exert a force which supplants that of the root under the given condi-tions. Although in some roots the rates are lower immediately after sever-ing the root from the shoot, fifteen hours later the rate of transport by theexcised root is frequently greater than that during a previous correspondingintact period. It has been shown (16), however, that rates at given axiallevels of young roots increase with time in both the intact and excised state.

Fluctuations in the characteristic gradients of water intake occur alongthe longitudinal axis of intact and excised onion roots. Spontaneous varia-tions in rates, sometimes opposite in direction, are also simultaneously ex-hibited by contiguous levels from one two-hour interval to another (15, 16).

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Water loss from epidermal cells has never been observed at any axial levelin the healthy growing onion root; but previous experiments have not shownwhether or not a decrease in rate was produced by liquid loss from minutecellular areas. The area at a single potometer contact in experiments onseveral hundred roots was seldom less than 2, nor greater than 4, squaremillimeters; if liquid loss occurred from minute cellular areas decreasingthe total intake at a single potometer contact, such loss was obscured by analgebraic over-all intake at a given level during the interval. JENNY,OVERSTREET, and AYERS (9) have demonstrated that intake of ions by someroots is not a unidirectional process and that the same ion species may moveinto and out of the root at the same time. Other ion exchange studies havealso been made; MAZIA (13) showed that ion exchange occurs in Elodea;MULLINS and BROOKS (14) and BROOKS (4) have demonstrated radioactiveion exchanges in single cells. Is water absorption a unidirectional process?Does water loss occur when rates of intake decrease and exudation pressurefalls? Does water loss enter into the maintenance of the "rhizosphere"around roots? SIERP and BREWIG (18) and BREWIG (2) have describedwater loss in the apical region of Vicia faba simultaneous with absorption inbasal regions.

The appearance of small irregular oscillations in the "bleeding" of cutstems with root systems attached was recorded by BARANETZKY in 1877 (1).In a recent article by HEYL (7) similar phenomena are mentioned. No mea-surements were made by either investiaator to determine the possibilities ofparallel variations in water intake and exudation. GROSSENBACHER (5) hasdescribed diurnal fluctuations in root pressure and volume exudation.Measurements of water intake to determine the presence or absence ofparallel recurring maxima and minima in both absorption and exudationwere not included in his investigations. In studies on the influence of theshoot on root permeability and resistance to water intake BREWIG used rootsystems with a portion of the stem attached (2). In a more recent articleBREWIG (3) describes experiments with isolated root segments, but the ob-ject of the experiments differed greatly from those which are presented inthis paper; BREWiG determined the effect of passing air over a portion of theroot segment to simulate transpiration; he also used osmotically active solu-tions. In the present investigation no attempt was made to determine theeffect of different agents on water transport. It appears to be the first studyto include simultaneous measurements of absorption in contiguous regionsand exudation in root tissue per se in a saturated atmosphere under constantexternal conditions. The technique used in the following experiments makespossible the simultaneous measurement of intake, outgo, and retention ofwater and therefore fulfills the fundamental experimental requirement inan adequate quantitative study of water balance.

448



ROSENE: WATER BALANCE IN THE ONION ROOT

Methods and resultsWATER TRANSPORT IN EXCISED ROOTS: RELATION OF VOLUME ABSORPTION TO

VOLUME EXUDATION

Calibrated potometers of small bore attached in a horizontal position toan upright glass rod within glass chambers provided a means of determininogabsorption rates and exudation in different root regions at the same time.Experiments were carried out in the dark with a saturated atmosphere in-side the chambers. Room temperature did not vary more than ± 0.50 C. inthe longest experiments. Condensed liquid which collected on the observa-tion window was removed before each reading by careful manipulation of a"window wiper" from the outside. Further details of the apparatus maybe obtained from previous publications (15, 16). Roots were obtained fromonions (Alliutrn cepa) grown in aerated nutrient solutions or in soil.

Simultaneous measurements of water intake and of exudation of over 100individual excised roots reveal that fluctuations in both volume of inflowand outflow occur from one two-hour interval to another and that the varia-tions in volume outgo may occur independently of variations in intake. Theratio of volume output (exudation) to volume input (absorption) duringany two-hour interval may be equal to, slightly greater, or less, than 1.Characteristic results are represented by the data in table I; additional datawould be repetitious.

The plants from which the data in table I were obtained were placed inthe experimental chambers the night preceding the experiment; excisionswere made the following morning without removing the chamber covers inorder to eliminate any marked change in humidity within the chambers.Only a portion of the potential absorbing surface of each root was utilizedsince not more than four potometers were used for absorption; the absorb-ingr area at a single potometer contact was not greater than 3.69 sq. mm. andinot less than 1.45 sq. mm. The potometers were spaced 10 mm. apart with thebottom potometer at the apex. Readings were made at 2-hour intervalsthroughout a 24-hour period following an initial reading at 10 A.M. Elon-gation at decreased rates continued in the excised roots from 6 to 12 hours.

All four roots manifested marked oscillations of the ratio of volume inflow(absorption) to volume outflow (exudation) from interval to interval. In-dependent fluctuations in both input and output were apparent; frequentlydiiiiinution of intake simultaneous with acceleration of outflow, or vice versa,was displayed.

The greatest range of variation of ratios appears in root II, which ex-hibited a ratio of 0.05 during the second and 1.05 during the tenth interval.All four roots manifested a ratio less than unity during the first interval.This is usually the case iimmediately following excision. Although precau-tions were taken to supply the plant with an abundance of water preceding

449



PLANT PHYSIOLOGY

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excision, it may be that the bulb with developing leaves produced a waterdeficit in the root tissue. Water deficits and the requirements of growthmight account for lower ratios during the first half of the 24-hour periodbefore growth had ceased and before a ratio of 1.0 was attained. Root III,however, exhibited equal outflow and inflow during the second, third, andfourth intervals when growth took place and saturation deficits may havebeen present. Root III is also an exception to the fact that in most casesthere is a marked increase in the magnitude of both inflow and outflow withtime. Many young roots manifested a sixfold to tenfold or greater volumeincrease with time during the 24-hour experimental period. Exudationusually reached a maximum before the 24-hour experimental period ended.Whether or not this behavior indicates a diurnal cycle has not as yet beendetermined. During consecutive two-hour intervals a twofold and even athreefold change in volume exudation was sometimes observed.

ABSORPTION AND EXUDATION OF RELATIVELY APICAL SEGMENTS

It was early observed that if an excised root was converted into an iso-lated segment by removal of the root cap region with a clean cut excision,exudation occurred solely at the basal end of the segment whether or notit was in an horizontal or a vertical position. When an excised root 65 mm.in length was cut in two, exudation appeared at both ends of the upper half.Individual experiments were made on 35 roots to determine the exact levelat which exudation appeared at both apical and basal ends of an intermediatesegment.

Measurements of simultaneous absorption and exudation were made onthe intermediate segments which were isolated by removing different lengthsof the apex and base of excised roots. The lengths of the tip removed dif-fered by increments of 0.5 to 16 mm. Five potometer tubes were used; thecut ends of the intermediate segments extended into tubes 1 and 5, and waterwas absorbed from tubes 2, 3, and 4 placed between them. The potometerswere 10 mm. apart. Owing to space limitations, data from only 9 roots aregiven in table II.

When less than 2 mm. was removed from the apical end of the excisedroot, the intermediate segment manifested elongation. This was due to thefact that the greatest amount of elongation in onion roots occurs in the secondmillimeter from the apex which in this case had not been removed. At thebeginning of the experiment a small drop of water was placed over each cutend in potometers 1 and 5; this was usually absorbed by the apical end asshown by segments A, B, and C in roots I, II, and III (table II), whichexhibited no exudation at the apical end. Segments with 5 to 16 mm. of thetip removed displayed absorption or exudation at the apical end duringalternate intervals. The magnitude of total absorption, however, exceeded

451



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that of total exudation in all but segments E and F from root V and VI.When more than 16 mm. was removed in roots of this age exudation usuallyexceeded absorption at the cut apical end.

The transport ratio of volume outflow to volume inflow total absortiontotal absorptionwas seldom unity during the first interval following excision. With timethe ratio approached or reached unity, the isolated segment transporting allthe water which was simultaneously absorbed; a ratio of one was not reachedby growing segments. A comparison of the ratios during equal intervals oftime and constant temperature as in segments C, D, and E from roots III,IV, and V in table II show that exudation may be equal to, slightly greater,or less than, absorption during consecutive intervals. In most cases in whichthe isolated segments were placed in the chambers the night before, the seg-ments manifested oscillating ratios during two-hour intervals the followingday but a ratio of unity when total outflow and inflow (sum of intervals) wascompared. In table II, segments F, G, and H show ratios of one during the12- or 14-hour period of the second or third interval; in this case, however,it is not known if the ratios were maintained from instant to instant.

The average rates of water intake from potometers 2, 3, and 4 were deter-mined but are not given in table II owing to space limitations. The highestrates appeared at the relatively more basal levels and fluctuations of ratesoccurred changing the gradient of distribution from time to time. Com-parisons of rates of water absorption of a given length of root when the rootwas intact (saturated atmosphere), when it had been excised, and finallywhen completely isolated from the root, showed that the given root tissuemanifested the same type of behavior it exhibited before it was cut fromthe excised or intact root.

DIRECTION OF TRANSPORT IN ISOLATED CONTIGUOUS SEGMENTS

Figure 1 is a diagrammatic representation of total outflow and of averagerates of intake of water in three segments cut from a 78-mm. root 5 days old.The segments were placed in a vertical position with 5 horizontal potometersattached, two (designated by arrows 1 and 5, fig. 1) to collect exudate andthree (designated by arrows 2, 3, and 4, fig. 1) filled with tap water. Thehighest average rate of intake appeared in the middle segment at a level 45mm. from the apex. Comparison of apical and basal outflow in each seg-ment showed that the direction of outflow in the apical third was entirelybasal against gravity; in the middle and basal thirds outflow occurred in twodirections; in both segments the basal outflow was greater. The total out-flow in the apical segment was relatively low, but in this case not only werethe rates of intake also lower but growth of the segment had taken place.Although the highest rate of intake occurred in the middle third, greater

4253



PLANT PHYSIOLOGY

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FIG. 1. Diagram of total volume outflow and average rates of intake of water inisolated contiguous segments of the same root. Positions of the potometers are desig-nated by the arrows which are numbered 1, 2, 3, 4, and 5. Observations were madeduring a 20-hour period.

quantities of water were absorbed and transported by the basal third whichwas greater in diameter and, therefore, the area of absorption at the 3potometers was correspondingly greater. The results obtained from thisroot are typical of segments of the same length and relative position cutfrom roots 75 to 100 mm. in length. Occasionally a basal segment exhibitedgreater exudation at the apical end of the segment but outflow occurred inboth directions. When, however, basal segments were isolated from olderand longer roots (over 200 mm. in length) and placed in their normal up-right position with respect to gravity the direction of transport in theupright position was toward the apical end.

454




Figure 2 shows volume outflow of longer segments from two older rootsdesignated as A and B. Observations were made during a 15-hour periodwith tap water in three potometers placed between those into which exuda-tion flowed. In both roots, the relatively more basal segments manifestedexudation at the apical end only. As indicated by the negative (-) signmost of the water covering the cut basal end disappeared. Since the vesselswere openi at both ends, this disappearance was caused by gravity, the waterappearing in the potometer at the apical end in each case. In root A. no

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FIG. 2. Comparison of volume exudation at the ends of isolated contiguous segmentsof two roots designated as A and B. Positions of potometers designated by arrows.

exudation appeared at the apical end of the apical segment; root B, on theother hand, manifested a small quantity of exudation at this end. Thesecond segment from the apex manifested exudation at both cut ends; thevolume flow in each direction was equal in root A and practically so inroot B.

The direction of outflow exhibited by the segments of the above two rootsis characteristic of segments cut from older and longer roots. When basalsegments were cut from roots 300 to 400 mm. in length and a drop of waterwas placed over each end, with the segment in the upright position, the waterfrom the basal end flowed down the vessels appearing in the bottom (apical)

4a;5



PLANT PHYSIOLOGY

potometer immediately after; placed in an inverted position, the reverse wastrue; placed in a horizontal position equal amounts of exudation appearedin potometers at the two ends of the segments when water was absorbedfrom potometers in between.

The direction of transport through the stele of an isolated segment de-pends upon its age and upon the level of the segment relative to its positionon the longitudinal- axis. Segments from the apical third of young roots lessthan 65 mm. in length and less than a week old exhibit basal outflow onlyin an upright, inverted, or a horizontal position; segments from the middleand basal third exhibit outflow at both ends in all three positions. In iso-lated pieces of roots of this length the direction of transport is chiefly basal,structural features apparently playing a dominant role. If, however, seg-ments are removed from levels near the bulb in roots over 200 mm. in lengthand 3 or more weeks old, the direction of transport depends upon the posi-tion of the isolated segment with respect to gravity. This indicates that thevector forces of transport are chiefly radial in isolated segments which arerelatively older whereas in younger segments both longitudinal and radialfactors are involved.

DiscussionIt is important to note that the present investigation is concerned with

volume transport and not pressures as such. Since external conditionswere carefully controlled, the factors which brought about variations in the

ratio of outfow had their origin in changes in the root itself. That theseinflowfactors are sharply localized is shown by the simultaneous changes in ratesat the different levels of water intake. The total intake during any oneinterval is a summation of the local changes. Whether cells gain or loosewater from one another is not shown but evidently the factors involved alterthe flux relations of water in individual cells. This indicates the unequaldistribution of forces (SHULL, 17) throughout the isolated root tissue underthe given conditions. The axial gradient of intake at any one moment isthe algebraic sum of individual cellular activities; it varies in relation toboth time and space under the particular conditions employed. The datafurnish no evidence to indicate whether or not the opposite variations ofoutflow and inflow are produced by separate mechanisms. It is not believedthat they are produced by faulty technique; similar experiments, however,should be carried out by other investigators to establish this property ofisolated roots.

With respect to water balance, determination of output, especially dur-ing short intervals, is not necessarily a measure of input and vice versa.This may be true in all types of roots; if so, conclusions regarding waterabsorption are not valid when based upon measurements of exudation and

456




not water intake itself. SPEIDEL (19) made parallel determinations of waterintake and exudation in the decapitated root system of Plectranthus. Hefound that volume intake was greater than volume outflow throughout a 24-hour period. Similar results have been obtained with onion roots in a fewexperiments. In most cases, however, where intake was greater than out-flow immediately after excision a ratio of unity was reached before 24 hourshad passed.

The magnitude of both volume inflow and outflow increased with timeafter excision. It may be that the change in rates with time indicated anobscure periodicity. BARANETZKY (1) observed two-hour oscillations in the"bleeding" of decapitated root systems in Ricinis insignia plants 2 weeksold with an absence of decided periodicity at this age; but when observationswere nmade on plants 5 weeks old, pronounced periodicity was noted.

Examination of GROSSENBACHER'S curves (6) obtained from toppedHelianthus plants shows two- and threefold variations of rates of outflowbetween maxima and minima of 24-hour cycles under constant external con-ditions. Throughout a 12-hour period isolated onion roots frequently mani-fest a sixfold increase in rate of outflow. KRAMER (11) attached the cutstems of exuding cotton stem-root systems to a vacuum pump and noted(p. 486) that "four or five times as much water exuded under the reducedpressure as had exuded during the same period of time due to 'root pressure'alone." This change is no greater than that observed in a similar 12-hourperiod with onion root tissue in the absence of applied suction; in the onionroot the change occurred under constant external conditions and was aproperty of the tissue itself; whether or not a similar capacity is present incotton roots has not been shown. By attaching topped tomato root systemsto a vacuum pump and lowering the pressure KRAMER (12, p. 787) "ap-proximately doubled" the rate of exudation. Again it is interesting to notethat single excised onion roots under constant conditions will do the samewithout artificially applied suction during consecutive two-hour intervals.KOHNLEIN (10) and others have also applied suction to topped plants andobtained an increase in exudation; it may be that a similar increase in mag-nitude would have been manifested by the excised root tissue over a periodof time under constant external conditions without the application of thesuction pump. BREWIG (3), however, calls attention to the gradual declineof rate of exudation following excision of roots of Vicia faba; he maintainsthat in his experiments the role of "bleeding" can be ascertained only ifmeasured immediately after cutting. When SPEIDEL made parallel mea-surements of water inflow and outflow in the decapitated root systems ofPlectranthus he obtained a sharp increase in both, followed by a progressivedecrease with time during a 24-hour period (19, fig. 23, p. 102). SPEIDELattributes the steep descent of both outflow and inflow and the absence of

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PLANT PHYSIOLOGY

periodicity to oxygen deficiency produced by a gradual decrease of oxygenin the closed potometers. Closed potometers were not used in the presentinvestigation.

JAMES and BAKER (8) described experiments with cut pieces of Sycamoreroots which manifested uptake of water by the morphologically lower endand exudation at the upper end. They maintain that "the exudation isalways in the morphologically upward direction even when the vessels havebeen opened at both ends, and will allow water to pass freely in the longi-tudinal direction. A drop of water placed at the physically upper end ofthe piece of root immediately causes a drop to appear at the lower end."Similar phenomena were not observed in pieces of onion root which mani-fested a longitudinal flow of water down the vessels when a drop of waterwas placed at the upper end of the root in a vertical position. Such piecesmanifest exudation at either the distal or proximal end depending uponwhether or not the segment is in an upright or inverted position; whenoriented horizontally, exudation appears at both ends. JAMEs and BAKERbelieved that their experiments furnished evidence that exudation from ves-sels did not take place. Although exudation from vertically placed basalsegments cut from relatively old roots appeared in the lowest of a series ofhorizontally arranged potometers, the present experiments on onion roots donot furnish evidence for or against the possibility of phloem exudation sinceexudation from the phloem at the upper end of the segment would flow downthe open vessels into the lowest potometer. The experiments do show, how-ever, that epidermal absorption takes place in the absence of vessels filledwith osmotically active solutes; forces outside of the vessels play a dominantrole in the mechanism of intake in this case.

BREWIG (2, 3) maintains that variations of intake observed in the dif-ferent root regions of Vicia faba are dependent upon transpiration. Asmentioned earlier he passed air over portions of isolated roots and rootsegments to simulate transpiration. The experiments on the onion rootshow that fluctuations of intake in isolated roots occur in a saturated atmos-phere under constant external conditions; in this case the regulatory phe-nomena are not associated with transpiration or evaporation from one endof the segment.

HEYL (7) has recently reviewed various theories dealing with the phe-nomena of "bleeding. " He discusses theories which deal with tissue poten-tials, electro-endosmotic flow, varying conditions of the plasma membrane,unequal osmotic forces at opposite sides of the cell, variations in the distri-bution of osmotically active substances in the cell wall, rhythmic pulsationprocesses in living cells, variations in the osmotic suction strength of thevessel liquid, expansion and contraction of the cell or vessel walls, local dif-ferences in the osmotic pressure at different regions of a tissue, and changes

458




in pressure in the vessels as a result of injury. Whether or not one or moreof the theories actually does apply to onion root tissue per se is only con-jecture at the present stage of the work.

Summary1. Simultaneous determination of water absorption and exudation of

excised roots and pieces of roots revealed that fluctuations in both inflowand outflow occurred from one two-hour interval to another in a saturatedatmosphere at a temperature which did not vary more than ± 0.50 C. duringlong intervals. The variations in outflow and inflow may be opposite indirection during any one interval.

2. The transport ratio of outflow to inflow, volume exudation was sel-volume absorption'

dom unity during the first interval following excision; with time the ratioapproached or reached unity. Rates of both inflow and outflow usually in-crease with time during a 24-hour period following excision.

3. The direction of longitudinal transport through an isolated piece ofroot depends upon its age and upon its position on the longitudinal axis be-fore cutting. Segments consisting of the apical third of young roots lessthan 70 mm. in length and less than a week old exhibit basal outflow only inan upright, inverted, or horizontal position with respect to gravity; segmentsfrom the middle and basal third exhibit outflow at both ends in all threepositions. The direction of transport of basal segments cut from roots over200 mm. in length and several weeks old depends upon the orientation of thesegment with respect to gravity.

4. Isolated segments of the root manifest the longitudinal gradient ofdistribution of rates of absorption characteristic of the excised and intactstate.

Ackknowledgment is made to A. A. HORAK for technical assistance.UNIVERSITY oF TEXAS

AUSTIN, TEXASLITERATURE CITED

1. BARANETZKY, J. Untersuchungen fiber die Periodizitiit des Blutens derkrautartigen Pflanzen und deren Ursachen. Abh. d. Naturforsh.Ges. Halle 13: 1-64. 1877.

2. BREWIG, A. Die Regulationserscheinungen bei der Wasseraufnahmeund die Wasserleitsgeschwindigkeit in Vicia faba Wurzeln. Jahrb.wiss. Bot. 82: 803-828. 1936.

3. . Ausl6sung leichter Wasserdurchliissigkeit an Wurzelnvon Vicia faba. Planta 29: 341-360. 1939.

4. BROOKS, S. C. Ion exchanges in accumulation and loss of certain ions

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PLANT PHYSIOLOGY

by the living protoplasm of Nitella. Jour. Cell. Comp. Physiol.14: 383-401. 1939.

5. GROSSENBACHER, K. A. Diurnal fluctuation in root pressure. PlantPhysiol. 13: 669-673. 1938.

6. . Autonomic cycle of rate of exudation of plants. Amer.Jour. Bot. 26: 107-109. 1939.

7. HEYL., J. G. Der Einfluss von Aussenfaktoren auf das Bluten derPflanzen. Planta 20: 294-353. 1933.

8. JAMES, W. O., AND BAKER, H. Sap pressures and movements of sap.New Phytol. 32: 317-343. 1933.

9. JENNY, H., OVERSTREET, R., AND AYERS, A. D. Contact depletion ofbarley roots as revealed by radioactive indicators. Soil Sci. 48:9-24. 1939.

10. K6HNLEIN, E. Untersuchungen uiber die Hohe des Wurzel-Wider-standes und die Bedeutung aktiver Wurzeltiitigkeit fur die Wasser-versorgung der Pflanzen. Planta 10: 381-423. 1930.

11. KRAMER, P. J. The intake of water through dead root systems and itsrelation to the problem of absorption by transpiring plants. Amer.Jour. Bot. 20: 481-492. 1933.

12. . The forces concerned in the intake of water by trans-piring plants. Amer. Jour. Bot. 26: 784-791. 1939.

13. MAZIA, D. The binding of Ca, Sr, and Ba by Elodea protoplasm. Jour.Cell. Comp. Physiol. 11: 193-203. 1938.

14. MULLINS, L. J., AND BROOKS, S. C. Radioactive ion exchanges in livingprotoplasm. Science n. s. 90: 256. 1939.

15. ROSENE, H. F. Distribution of the velocities of absorption of water inthe onion root. Plant Physiol. 12: 1-19. 1937.

16. . Comparison of rates of water intake in contiguous re-gions of intact and isolated roots. Plant Physiol. 16: 19-38. 1941.

17. SHULL, C. A. Imbibition in relation to absorption and transportationof water in plants. Ecology 5: 230-240. 1924.

18. SIERP, H., AND BREWIG, A. Quantitative Untersuchungen uiber dieWasserabsorptionszone der Wurzeln. Jahrb. wiss. Bot. 82: 99-122. 1935.

19. SPEIDEL, BERTHOLD. Untersuchungen zur Physiologie des Blutens beihoheren Pflanzen. Planta 30: 67-112. 1939.

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