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The transition from an aquatic environment to a terrestrial one involves many adaptive changes. To set the stage for a more careful examination of that transition, we can start by looking at those features common to land plants.

1.         There must be protection against evaporation. This, probably the most obvious adaptation, has been achieved by the development of epidermal tissue and, for some structures, a special waxy layer or cuticle external to the epidermis or outermost plant tissue. Delicate tissues, such as leaves, most need the epidermis and its cuticle. Other structures, such as roots and stems and their counterparts in the less complex land plants, are effectively protected by the epidermis. The bark of trees is an obvious example.

3.         Gaseous exchange with the environment must be possible. The leafy tissues must not be so completely covered that the release of oxygen and the uptake of carbon dioxide is prevented. Such a two-way flow of gases is essential to life. In the leaves of higher plants this flow is facilitated by special structures called stomata. Under conditions of low humidity, cells on either side of the stomatal opening expand and effectively close the opening. Under other conditions, and depending on the needs of the nearby tissues, the stomata are opened to varying degrees.

4.         Water must be absorbed. Roots or root-like structures called rhizoids perform this function. The actual absorption by roots is carried out by root hairs. These microscopic cells extend at right angles from the root surface and provide the cell surface needed to absorb water and salt and other dissolved nutrients that may be present in the water.

5.         Materials must be transported throughout the plant body. Photosynthetic products from leafy parts must be available to stems and roots and materials absorbed by roots must be available to stems and leaves. In the more complex land plants, special cells, which constitute the vascular tissue, conduct nutrients throughout the plant. Many land plants lack vascular tissue, but nonetheless transport is achieved by more generalized tissues. But since aquatic plants are surrounded by water, the plant tissues can exchange materials directly with the water, and there is no need for vascular tissue in the algae.

6.         Land plants need support to keep them upright. Water is a buoyant medium and in its absence algae collapse into rather pathetic heaps. The special development of tough cell walls is used by plants for support. Additionally, some stem cells are specialized; there, the fiber cells carry the weight of plants. Consider, in particular, a giant redwood; it holds up thousands of tons, for centuries.

7.         The gametes and especially, the early stages of new generations, must be protected. The terrestrial environment can be a relatively hostile place in which to germinate and survive. Spores and in particular seeds, which carry embryonicplants, are adapted to survive dry conditions and to respond to wet conditions by germination and subsequent growth.