34 THE PLANT WORLD. 



modification of the relative speed or precession of the various 

 processes in the gamete, or of the activities of the chain of en- 

 zymes, or of some disturbances of the delicate balance among 

 the ions in the protoplasm. Still beyond these, stimulative 

 responses or reactions may occur, which would find their ulti- 

 mate expression in deviations in external form of the progeny 

 of an individual subjected to the unusual agencies in ques- 

 tion. Changes of the same nature may well be induced by 

 many conditions to which plants are naturally subject, the 

 survival of the resultant forms being solely a matter of pos- 

 sible interbreeding and habital selection. 



A stream takes its rise near the alpine plantation of the 

 Desert Laboratory, and flows out on the desert a few miles 

 away, and a mile lower down. Doubtless hundreds and 

 thousands of seeds are carried to the lowlands each year. 

 Some of these develop into individuals which carry out re- 

 production. This is usually done in the native habitat, at 

 actual temperatures of the tissues not above 60^ or 70° F. 

 Down below, spore formation, reduction divisions and fer- 

 tilization may ensue in temperatures 40 or 50 degrees higher, 

 a difference capable of being endured by the shoots of some 

 plants now being tested, and which might well cause irre- 

 versible developmental changes. Other factors of the en- 

 vironment may operate in a similar manner. 



Perhaps the most evident opportunity offered to the 

 student of the desert is that which promises a solution of 

 some of the problems of distribution, particularly with re- 

 spect to plants. The absence of crowding of the elements in 

 a formation, the definiteness of the factors affecting move- 

 ments, and the great areas which can be brought under ob- 

 servation in which no disturbances by cultivation have arisen, 

 lay the problem out with the clearness of a diagram. A day 

 spent in sectioning the vegetation of a mesa and an adjoining 

 mountain slope will show that the conventionalized comments 

 customarily given in descriptive v.'orks as to centers of dis- 

 tribution, place or origin, significancy of density of popula- 



