78 



The Plant World. 



in climatic conditions toward greater dryness would result 

 in eliminating the palma samandoca from the footslopes. 



There is, however, a certain difficulty involved in the above 

 explanation, which must not be minimized. Why do not the 

 supposedly more resistant plants of the south slope avail them- 

 selves, equally with those of the north slope, of the favorable 

 conditions for germination? If the case is as has been stated, 

 we should rather expect that the vegetation of the north slope 

 would be composed of all the plants of the locality, while that 

 of the south slope would consist of the types with quickly re- 

 sistant seedlings. The answer may very well be that the course 

 of events is bringing this about, and that we are beholding a 

 stage in the process, and this view receives support from the 

 fact that, as above stated, the species are not confined entirely 

 to their apparently appropriate slope, and, further, my notes 

 indicate that more south slope plants are found on the north 

 slope, than north slope plants on the south slope, though this 

 statement needs verification. It is not without significance 

 that, at elevations above the zone in which the ocotillo occurs, 

 the palma samandoca passes to the south slope, where, at the 

 elevation of 8,000 feet the conditions appear to approach those 

 of the north slope at lower levels. It is not, however, unlikely 

 that some of the plants of the south slope find that the soil 

 moisture of the north slope, meagre as it may be, is above their 

 optimum. Per contra, some succulents (certain cacti) are to 

 be found in habitats where soil moisture is relatively abundant. 

 And it must not be forgotten that competition between individ- 

 uals is not a negligible factor in the region in question, where 

 the ground is normally quite crowded with perennial vegetation, 

 and this, it appears probable, must enter into a final analysis 

 of the situation. For the present the problem here stated 

 serves to emphasize the necessity of exact observations of the 

 moisture conditions of both the soil and of the atmosphere, 

 and of the further desirability of correlating such observations 

 with the peculiarities of the life histories of each particular plant 

 in question. It is not going too far to say that, in the final 

 analysis, the problem of the desert may be understood only when 

 the physiological peculiarities of each plant have been thoroughly 

 studied. 



Alabama Polytechnic Institute. 



