44 



THE AMERICAN NATURALIST 



[Vol. XL 



than one fourth the entire transpiring surface, the leaves alone 

 transpired nearly one half the whole amount. 



The high humidity at this time was unfavorable to the further 

 use of the polymeter method so that the experiments upon the 

 transpiration of the leaves of the cactus were continued with a 

 special weighing apparatus in its stead. Since the results of all 

 of these experiments were essentially alike, I shall refer to one of 

 them only. 



A branch of Opuntia versicolor with leaves was placed in a 

 bottle containing water which was so arranged, with a capillary 

 tube as well as the branch fastened in the stopper, that the air 

 could enter and maintain a pressure within uniform with that of 

 the room, while only an inappreciable quantity of vapor escaped. 

 In one hour, 2 : 15 to 3 : 15 p. m., the branch lost 180 milligrams 

 in weight. The stem was then coated with vaseline and in one 

 hour, 3 : 30 to 4 : 30 P. m., the loss of weight was 100 milligrams, 

 which was, of course, the transpiration of the leaves only. 



There were 69 leaves on the branch whose entire surface was 

 estimated at 55 sq. cm. The surface of the stem alone was 65 

 sq. cm. Therefore the leaves had about 45 percent of the entire 

 transpiring surface and they gave off about 55 percent of the 

 entire amount transpired. 



Sl-MMARY AND CONCLUSIONS 



The leading points in this paper and the conclusions may be 

 briefly stated in the following summary. 



1. The root systems of Cereiis gicfanteus and of Echinocadus 

 wislizeni which were studied and mapped, present characteristic 

 differences. The root system of Cereus is in part superficial 

 and in part deeply placed. The root system of Echiiioeactus is 

 superficial only. There aj)i>cars to he a relation l)et\ve(Mi tli(> eliar- 

 acter of the root systems ui t'le^e plants and that of the habitats 

 in which they naturally occur. For example, the i"oriii and the 

 extension of the roots of Cereus inhibit its occurrence in localities 

 where the tmdedying formation is of such nature that thev cannot 

 reach the usual or needful depth. We accordingly find the plant 

 on rocky mountains, or where the soil is deep, but in this locality 



