THE PLANT KINGDOM 253 



provides the only source of water available to a land plant, it is 

 evident that roots or root-hke structures must be developed to 

 penetrate the soil and absorb water therefrom abundantly. 

 A successful accomplishment of photosynthesis requires a large 

 area of chlorophyll exposed to sunlight, and hence broad sheets 

 of chlorophyll-bearing tissue, "waterproofed" to prevent undue 

 evaporation, must also be evolved. These sheets we call leaves. 

 The leaves cannot be tdo close together without depriving one 

 another of the necessary light, and they must therefore be spread 

 out and separated in some way on an axis or stem. The region 

 where water is constantly needed to replace water loss may thus 

 be far distant from the region where it is absorbed, and a well 

 developed conducting system to carry water from root to leaf 

 must therefore be differentiated in the tissues of the stem. 



Aside from the difficulty of maintaining a sufficient supply 

 of water, the land plant also faces problems of a mechanical nature. 

 Owing to the buoyancy of water, a plant growing submersed 

 therein needs little or no mechanical support. If it grows in the 

 air, however, there is much weight to be carried and a heavy 

 strain to be borne by the stem, especially in its lower portions. 

 An extensive development of thick-walled skeletal and supporting 

 tissue is thus necessary, especially in the stem, if the plant is to 

 be kept firm and erect. 



In order to be able to thrive on land a plant must therefore 

 possess successfully functioning roots and leaves, and a stem 

 able to serve as an' efficient means of conduction and support. 

 Such structures are unknown in the Thallophytes, and these 

 plants have therefore never been able to invade the dry land and 

 to produce a true terrestrial vegetation, although they often 

 thrive in moist situations on land and survive long periods of dry- 

 ness in a dormant state. The first group to emerge from the 

 water and develop land-inhabiting forms were probably the 

 Bryophytes or plants like them, which may perhaps be called 

 the "amphibians" of the plant world. They are best developed 

 in moist places, though a few are aquatic and many grow in 

 situations which are dry much of the time. Even in the most 

 highly developed mosses, however, the root system is very weak 

 and consists only of delicate thread-like rhizoids; the leaves are 

 small and very thin, and the stems weak and with little or no 

 development of supporting and conducting tissue. The mosses, 



