ECOLOGY 143 



With this change in habit came a need of taking in water, of 

 storing it, of conducting it to various parts of the organism. So 

 it does not seem unlikely that plants came to have roots, 

 stems, and leaves and thus adapted to their environment on 

 dry land. We find in nature that those plants or animals which 

 are best adapted or fitted to live under certain conditions are 

 the ones which sur\dve or drive other competitors out from 

 their immediate neighborhood. Nature selected those which 

 were best fitted to live on dry land, and those plants eventually 

 covered the earth with their progeny. 



As we have found in our experiments, young plants, and indeed 

 any living plants, are delicate organisms, which are affected pro- 

 foundly by the action of forces outside themselves. It is impossi- 

 ble not to see this after we have grown seedlings with and with- 

 out light, with much water and with little water. Pea seedlings 

 may grow for a time in sawdust, but we know that they will be 

 much healthier and will live longer if allowed to germinate in 

 soil under natural conditions. 



Desert Conditions. — If we examine plants growing in a dry 

 climate, as cactus, sage brush, aloe, etc., we find that the leaf 

 surface is invariably reduced. Leaves are reduced to spines in 

 the cactus. Some plants, such as the three-angled spurge, which 

 bear leaves in a condition of moderate water supply, take on 

 the appearance of a cactus under desert, conditions. Thus they 

 lose their evaporating leaf surface by having the leaves changed 

 into spines. 



This adaptation is evidently, if our experiments count for 

 anything, the result of the action of forces outside the plants; 

 that is, it is an adaptation to environment. 



Water Supply. — Water supply is one of the important factors 

 in causing changes in structure of plants. Plants which live en- 

 tirely in the water, as do many of the plants known as algse, have 

 slender parts, stemlike, and yet ser^nng the place of a leaf. The 

 interior of such a plant is made up of spongy tissues which allow 

 the air, dissolved in the water in which they live, to reach them. 

 If leaves are present, as m the pond lily, the stomata are alJ in 

 the upper side of the leai: 



