Adaptations For Life On Land
Much evidence suggests that plants evolved from freshwater green algae (Figure10.3). For
example, green algae and plants both have the carbohydrate cellulose in their cell walls and
they share many of the same pigments. (For a review of plant cells, see theCells and Their
Structureschapter.) So what separates green algae, which are protists, from green plants?
Figure 10.3: The ancestor of plants is green algae. This picture shows a close up of algae on
the beach. ( 11 )
One of the main features that distinguishes plants from algae is the retention of the em-
bryo during development. In plants, the embryo develops and is nourished in the female
reproductive structure after fertilization. Algae do not retain the embryo. This was the first
feature to evolve that separated the plants from the green algae. Plant reproduction will be
discussed in the following section.
Although the retention of the embryo is the only adaptation shared by all plants, over time
otheradaptationsforlivingonlandalsoevolved. Inearlyplants,awaxylayercalledacuticle
evolved to help seal water in the plant and prevent water loss. However, the cuticle also
prevents gases from entering and leaving the plant easily. Recall that the exchange of gasses -
taking in carbon dioxide and releasing oxygen - occurs during the process of photosynthesis.
Therefore, along with the cuticle, small pores in the leaves calledstomataalso evolved
(Figure10.4). The stomata can open and close depending on weather conditions; when it’s
hot and dry the stomata can stay closed to conserve water. The stomata can open again to
permit gas exchange when the weather cools down.
A later adaption for life on land was the evolution of vascular tissue. Vascular tissueis
specialized tissue that transports water, nutrients, and food in plants. In algae, vascular