32 THE PLANT WORLD. 



It is In this period that the action of the major factors 

 upon living things in the formation of deserts will be simu- 

 lated. Hundreds of species will be subject to unusual condi- 

 tions of soil-moisture, saline solutions and seasonal change, 

 while variations of temperature of wide amplitude and other 

 possible inciting causes may exert the strongest influence upon 

 their ontogenetic development. The question as to the na- 

 ture of possible adaptations of plants under such conditions 

 presents itself at once. 



It seems to be tacitly assumed by the majority of writers 

 for example, that the changes undergone by a mesophytic 

 species when grown in an arid situation are of such nature as 

 to fit the individual better for living under the drier condi- 

 tions. This does not always follow. The form assumed by a 

 shoot in a dry atmosphere is the result of growth conditions 

 established by the balance among the absorbing organs, the 

 conducting stems, and the transpiratory surfaces. In many 

 cases the restricted surfaces are an advantage, but it is an 

 advantage offset by the fact that the root-systems have equal- 

 ly failed to accomplish the normal amount of growth so 

 that the general relations of the individual remains as before. 



Over a hundred species of seed plants were grown in 

 complete darkness and under faint illuminations in the experi- 

 ments upon etiolation which were completed three years ago. 

 The absence of light was seen to set in action morphogenic 

 processes of the most diverse character in various species, in 

 some of which the stems were enormously thickened, in oth- 

 ers elongated, with various unusual and totally useless devel- 

 opment of tissues, in which the supply of reserve food was 

 used. Illustration of these non-adaptive responses might be 

 multiplied indefinitely. 



Not only are the direct ontogenetic and morphogenetic 

 responses of plants to environmental forces not necessarily 

 of an adaptive character, but it is impossible to derive some 

 of the most highly specialized and heritable structures with 

 the supposedly causal conditions which they meet. Spines 



