252 BOTANY: PRINCIPLES AND PROBLEMS 



phytes) to those in which the gamete-plant alternates with a 

 small, dependent, spore-bearing structure, the primitive sporo- 

 phyte (in the Bryophytes); thence to forms where sporophjiie 

 and gametoph}^e are both independent plants but where the 

 former is now the large and conspicuous member (in the Pterido- 

 phytes); and finally to those in which the gametophyte is the 

 dependent and subordinate generation, and where the only plant 

 which we know as such is the spore-plant (in the Spermatophytes). 

 A knowledge of this progressive development will not only aid 

 us in understanding the process of reproduction in plants but is 

 perhaps the best approach by which we can gain a clear concep- 

 tion of some of the important distinctions between the four great 

 divisions of the plant kingdom. 



5. The Invasion of the Land. — The fifth great forward step in 

 plant history was the evolution of a type able to grow in the air 

 rather than in the water, and which thus made possible an 

 invasion of the dry land and the establishment there of a real 

 terrestrial vegetation. We have said that plant life probably 

 began in the sea. Here also doubtless took place the first great 

 steps in the evolution of the vegetable kingdom; and although 

 the seas teemed with life, the land masses of our earth were for a 

 very long period of time barren wastes, or at best covered in 

 their damper spots only with a scum of algae. This great 

 area was freely open to whatever plant pioneer should be able to 

 master the difficulties of such an environment. 



Difficulties of Terrestrial Life. — These difficulties were many 

 and formidable. First and most serious among them was the 

 problem of maintaining, in such dry surroundings, a sufficient 

 supply of water for protoplasmic activity. We have discussed 

 in an earlier chapter the supreme importance of water in the life 

 of plants, and have shown how indispensible it is in all physiolog- 

 ical processes. When the whole plant body is immersed in water, 

 as is the case in primitive and lowly forms, an ample supply of 

 this substance is always at hand. If a plant part is lifted up into 

 the air, however, it is at once exposed to the danger of water- 

 loss through evaporation, which will soon result in death. This 

 danger of drought has always faced plants which grow upon the 

 land. If a plant is to succeed in such an arid environment it 

 must be able both to absorb water in large amounts and to hinder 

 the loss of water from its tissues by evaporation. Since the soil 



