THE PLANTS STUDIED. 7 



In nearly all instances the experiments herein presented were repeated at, 

 roughly speaking, yearly intervals, and in this way a valuable check upon 

 all the work was obtained. If, as it may appear, but a beginning has been 

 made toward the solution of the difficult problems which have been attacked, 

 I may still venture to hope that the results will prove to be a stimulus to 

 further studies in the same direction. If this has been accomplished, it is 

 due in no small part to the personal concern which my colleagues have shown 

 in this work, and to them, as well as to the Institutions mentioned above, 

 I wish at this point to express my most earnest appreciation. Of these, I 

 would especially mention Dr. Daniel Trembly MacDougal, who has through- 

 out taken a most generous interest in my studies. The moral support of 

 one's co-workers is frequently no small element in his success. I wish also 

 to thank Prof. E. M. Blake for his help in making certain mathematical 

 interpretations used in the section on transpiration and stomatal action. 



THE PLANTS STUDIED. 



The material which has been used has been obtained chiefly from two 

 plants, the ocotillo (Fouquieria splendens, plates i and 2) and a showy desert 

 verbena (Verbena ciliata, plate 3), both of which grow abundantly upon 

 Tumamoc Hill, upon which the Desert Botanical Laboratory stands. 



FOUQUIERIA SPLENDENS* Wisl. 



The ocotillo (Fouquieria splendens, plates 1 and 2) is one of the most con- 

 spicuous and characteristic among the plants of the formation in which it 

 occurs. Its common name, "coach-whip cactus," conveys to the mind two 

 ideas — that the stems appear as a cluster of coach whips, lithe, slender, and 

 unbranched, and so standing at an angle with the ground as to suggest the 

 whip held in the hand, and that it looks much like the cacti which are 

 associated with it in its habitat, the likeness to which is strengthened by the 

 occurrence of thorns of remarkable morphological origin (Robinson, 1904). 

 Nor is this likeness merely superficial, since even a cursory examination of 

 the anatomical structure discovers the occurrence of water-storage tissue, the 

 cortex, the character of which brings the ocotillo very close to the cacti 

 physiologically. Another interesting feature of this plant, in which the like- 

 ness is rather to certain other types of desert forms, is the ready manner in 

 which it develops a complement of foliage. So readily does this occur that 

 the growth of leaves may be induced during the course of a few days merely 

 by sufficiently wetting the stem (Lloyd, 1905). Normally a full growth of 

 leaves follows an abundant shower, and accordingly the foliage is present 

 during the two periods of rainfall. It is, however, capable of responding 



*Wislezenus: Tour N. Mex., 98. 



