The Evolution of Plants

Did you know that plants used to live only in water millions of years ago? In fact, they didn’t even look like plants… plants

ancestors were simply algae! Eventually, they evolved to find their way onto land and make use of a completely new

environment.

Advantages for plants to live on land:

Living on land offered new opportunities such as…

o Unlimited sunlight

o Abundant CO2

o Initially, there were very few pathogens and herbivores.

Challenges for plants to live on land:

Even though living on land had its perks… there were also several problems with it…

Water beads on the waxy cuticle of kale leaves

Plants had to figure out how to regulate water loss within the cells. The waxy covering on top of leaves, called a plant cuticle,

evolved on the surface of leaves to reduce water loss. This is why leaves are shiny, waxy and water rolls off. Human epidermis

(skin) is similar in its function because our skin serves as a defense against physical damage and infectious organisms and the

oils on our skin help us retain water (and keep the epidermis flexible).

Plants had to figure out how to obtain resources from soil and air. There were no vessels that could carry water throughout the

plant… so eventually they developed vascular tissue to transport water/sugar within the plant.

Plants had to figure out how to support their body up in the air because algae had no

such structures. This gave way to the specialization of the root system and the shoot system.

Plants had to figure out how to reproduce and disperse their offspring without water. This gave way to spores, seeds and fruits

for dispersal and pollen for fertilization.

In all plants, the zygote develops into an embryo while attached to and nourished by the parent plant. Plants are embryophytes,

with multicellular, dependent embryos.

From algae to moss to seedless, vascular plants to seed-bearing plants to finally… flowering-plants.

The first group of plants that made it on land are best known as BRYOPHYTES.

Bryophytes: the

first land plants. They need relatively moist/wet environments to survive.

Early land plants reproduced with spores that would swim (with little whipping tails called flagella) through moist soil and find the

female organs. This is why they needed to be in constantly damp environments and early plants only existed near shores and

streams.

Without a robust vascular system, plants couldn’t get taller than a couple feet max and most were very close to the

ground. Plants eventually evolved to have vessels and those were able to grow very tall and grab more sunlight for

photosynthesis (literally overshadowing the competitors) and their spores were able to be blown greater distances due to the

greater starting heights.

With vascular tissue,

the plants could finally circulate resources throughout the plant. This allowed them to evolve to much larger sizes but they were

still seedless and relied on spores.

Seedless, vascular plants such as ferns formed vast ancient forests in low-lying wetlands during the Carboniferous period (360–

299 million years ago).

These ancient forests of ferns gave us modern day coal. In the middle of this period, a massive extinction event occurred

due to weather changes. When these ferns died, the plants formed peat deposits that eventually formed coal.

Coal, oil, and natural gas are fossil fuels. Oil and natural gas formed from marine organisms; coal formed from seedless

plants. Random fact: Insects used to grow much larger because the amount of oxygen in the environment was 40% (double of

today).

Depiction of what

an ancient forest looked like.

Then

gymnosperms came about which were the first to actually have vascular tissue AND SEEDS.

Seeds increased the success rate of the plants because they contained endosperm(nutrients packaged in with the embryo) that

gave the seeds the initial resources they needed to reach a decent size to survive.

Seeds are also important because they could be completely isolated from the external atmosphere and were protected from

desiccation (dryness), so they could be dormant for long periods of time during droughts and effectively be less vulnerable to

extinction. They can literally ride-out the toughest periods and leap back to life when conditions are good. The first seed bearing

plants, like the ginkgo and conifers (such as pines and firs), did not produce flowers.

Jacaranda mimosifolia: A

beautiful example of a flowering plant.

Angiosperms are flowering plants and are the most successful and diverse of the land plants. Flowers are highly-modified

leaves whose main point is for reproduction. Another blog post will be on the way about how they coevolved together with

insects.

Plant Evolution!

Plants adapted to the dehydrating land environment through the development of

new physical structures and reproductive mechanisms. LEARNING OBJECTIVES[ EDIT ]

Discuss how lack of water in the terrestrial environment led to significant adaptations in plants

Describe the life cycle of a haplodiplodontic species

KEY POINTS[ EDIT ]

While some plants remain dependent on a moist and humid environment, many have adapted to a more arid climate by developing tolerance or resistance to drought conditions.

Alternation of generations describes a life cycle in which an organism has both haploid (1n) and diploid (2n) multicellular stages, although in different species the haploid or diploid stage can be dominant.

The life on land presents significant challenges for plants, including the potential for desiccation, mutagenic radiation from the sun, and a lack of buoyancy from the water.

TERMS[ EDIT ]

desiccation tolerance the ability of an organism to withstand or endure extreme dryness, or drought-like condition

alternation of generation the life cycle of plants with a multicellular sporophyte, which is diploid, that alternates with a multicellular gametophyte, which is haploid

Plant Adaptations to Life on Land As organisms adapted to life on land, they had to contend with several challenges in the terrestrial environment.

The cell's interior is mostly water: in this medium, smallmolecules dissolve and diffuse and the majority of the

chemical reactions ofmetabolism take place. Desiccation, or drying out, is a constant danger for organisms

exposed to air. Even when parts of a plant are close to a source of water, the aerial structures are prone to

desiccation. Water also provides buoyancy to organisms. On land, plants need to develop structural support in

a medium that does not give the same lift as water. The organism is also subject to bombardment by mutagenic

radiation because air does not filter out the ultraviolet rays of sunlight. Additionally, the male gametes must

reach the female gametes using new strategies because swimming is no longer possible. As such, both gametes

and zygotes must be protected from desiccation. Successful land plants have developed strategies to face all of

these challenges. Not all adaptations appeared at once; some species never moved very far from the aquatic

environment, although others went on to conquer the driest environments on Earth.

Despite these survival challenges, life on land does offer several advantages. First, sunlight is abundant. Water

acts as a filter, altering the spectral quality of light absorbed by the photosynthetic pigment chlorophyll. Second,

carbon dioxide is more readily available in air than water since it diffuses faster in air. Third, land plants

evolved before land animals; therefore, until dry land was also colonized by animals, no predators threatened

plant life. This situation changed as animals emerged from the water and fed on the abundant sources

of nutrients in the established flora. In turn, plants developed strategies to deter predation: from spines

and thorns to toxic chemicals.

Early land plants, like the early land animals, did not live far from an abundant source of water and developed

survival strategies to combat dryness. One of these strategies is called desiccation tolerance. Many mosses can

dry out to a brown and brittle mat, but as soon as rain or a flood makes water available, mosses will absorb it

and are restored to their healthy green appearance. Another strategy is to colonize environments where

droughts are uncommon. Ferns, which are considered an early lineage of plants, thrive in damp and cool places

such as the understory of temperate forests. Later, plants moved away from moist or aquatic environments and

developed resistance to desiccation, rather than tolerance. These plants, like cacti, minimize the loss of water to

such an extent they can survive in extremely dry environments.

The most successful adaptation solution was the development of new structures that gave plants the advantage

when colonizing new and dry environments. Four major adaptations are found in all terrestrial plants:

the alternation of generations, asporangium in which the spores are formed, a gametangium that

produces haploidcells, and apical meristem tissue in roots and shoots. The evolution of a waxy cuticleand a cell

wall with lignin also contributed to the success of land plants. These adaptations are noticeably lacking in the

closely-related green algae, which gives reason for the debate over their placement in the plant kingdom.

Alternation of Generations

Alternation of generations describes a life cycle in which an organism has both haploid and diploid multicellular

stages (n represents the number of copies of chromosomes) . Haplontic refers to a lifecycle in which there is

a dominant haploid stage (1n), while diplontic refers to a lifecycle in which the diploid (2n) is the dominant life

stage. Humans are diplontic. Most plants exhibit alternation of generations, which is described as

haplodiplodontic. The haploid multicellular form, known as agametophyte, is followed in the development

sequence by a multicellular diploid organism: the sporophyte. The gametophyte gives rise to the gametes

(reproductive cells) by mitosis. This can be the most obvious phase of the life cycle of the plant, as in the mosses.

In fact, the sporophyte stage is barely noticeable in lower plants (the collective term for the plant groups of

mosses, liverworts, and lichens). Alternatively, the gametophyte stage can occur in a microscopic structure,

such as a pollen grain, in the higher plants (a common collective term for the vascular plants). Towering trees

are the diplontic phase in the life cycles of plants such as sequoias and pines.

Alternation of generations of plants

Plants exhibit an alternation of generations between a 1n gametophyte and 2n sporophyte.

Protection of the embryo is a major requirement for land plants. The vulnerable embryo must be sheltered

from desiccation and other environmental hazards. In both seedless and seed plants, the female gametophyte

provides protection and nutrients to the embryo as it develops into the new generation of sporophyte. This

distinguishing feature of land plants gave the group its alternate name of embryophytes.