. Principles of modern biology. Biology. soil and helped to prepare the land envi- ronment for the coming of other plants. SUBKINGDOM EMBRYOPHYTA Essentially the various groups of Thallo- phyta, which have been described in the foregoing sections, are relatively simple water-dwelling plants, despite the fact that a few species show some slight degree of adaptation to life upon the land. The Thal- lophyta appear to have originated quite early in the Proterozoic era (Fig. 29-10), at which time no terrestrial plants existed. Land plants, in fact, did not begin to appear until well after the early
. Principles of modern biology. Biology. soil and helped to prepare the land envi- ronment for the coming of other plants. SUBKINGDOM EMBRYOPHYTA Essentially the various groups of Thallo- phyta, which have been described in the foregoing sections, are relatively simple water-dwelling plants, despite the fact that a few species show some slight degree of adaptation to life upon the land. The Thal- lophyta appear to have originated quite early in the Proterozoic era (Fig. 29-10), at which time no terrestrial plants existed. Land plants, in fact, did not begin to appear until well after the early days of the Paleozoic era. At this point, therefore, the question of how plants became adapted to terrestrial condi- tions becomes pertinent. Adaptation of Plants to the Land Environment. Land-adapted nutritional organs, such as the leaves, roots, and stems of higher plants, did not spring suddenly full-fledged into being; and the same is true of the specialized tissues such as the conducting units of the xylem and phloem, and the well-developed water-proofed epidermal tissues, which are so essential to well-adapted land species (Chap. 13). It is interesting to note, there- fore, how these nutritional adaptations did arise gradually in the various groups of higher plants. The land environment also imposes two serious problems upon the reproductive mechanisms of the plant. If the plant is not submerged in water: (1) How can the sperm manage to swim to the eggs? and (2) How can the unicellular zygote manage to survive while the new plant is developing its first roots, stem, and leaves? No truly successful land species could appear, in fact, until the reproductive mechanisms became adjusted to meet these basic problems. The first problem did not reach a truly satisfactory solution in any of the earlier groups. Consequently the more primitive land species are restricted, more or less com- pletely, to areas where, at least periodically, there is an abundance of water in which the s
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