PROBLEMS OF THE DESERT. 33 



and spiculae do lessen the ravages of grazing animals upon 

 cacti to some extent, but these structures are undoubtedly a 

 direct and accumulated response to aridity, or poverty of 

 available water, and in a dozen species the deterioration of 

 leaves and branches has been carried so much farther that 

 we have that many spineless or nearly unarmed forms in 

 American deserts, which suffer variously or not at all from 

 animals. . Illustrations of a similar nature might be multi- 

 plied indefinitely. 



The entire matter rests upon the fact that when a 

 stimulus consisting of a change in intensity of temperature, 

 light, soil-solutions, or humidity is brought to bear upon a 

 plant it responds as a living machine, while at the same time 

 the direct effect of the external force upon the parts of the 

 machine or its processes may materially modify their prop- 

 erties, or cause radical alterations in the processes them- 

 selves. 



Another aspect of the subject remains to be considered. 

 In addition to the direct physiological, somatic response of 

 a plant expressed in its roots, stems or leaves, it is to be borne 

 in mind that many of the agencies which cause these varia- 

 tions also act directly upon the germ-plasm, which give a 

 direct response, and not of an adaptive nature. 



By the controlled application of climatic factors, Tower 

 has been able to induce the appearance of strains of beetles 

 which diverged from the parental type in one main character 

 with correlated variability in others, which crossed readily 

 with the parent and might be swamped by it. By the appli- 

 cation of reagents to the reproductive elements of seed- 

 plants I have been able to cause the appearance of new forms 

 of plants which diverge from the parent in several qualities, 

 which do not readily cross with the parent, and the newly 

 acquired characters are irreversible so far as the tests of 

 three generations may be taken as conclusive. Results of a 

 similar nature have been obtained by Gager with the use of 

 radium preparations. The direct effects might consist in a 



