114 THE PLANT WORLD. 



one of 0. versicolor, and the tliird was a small MammiUaria. 

 The curves of relative transpiration are seen to be practically 

 identical, all having a maximum in the night and a minimum in 

 the day, and thus agreeing with the generalization for cacti 

 stated above. The curves of absolute transpiration, on the other 

 hand, do not show any uniformity, and bring out clearly how 

 useless these curves are in a study of the physiological regulation 

 of water loss. 



In order to bring out the essential difference between the 

 curves of cacti and those of ordinary leafy plants, the curves 

 from a test of Verbena ciliata are presented in Figure '2. Here 

 relative transpiration is seen to be high by day and low l)y night, 

 the opposite condition from that prevailing in the cacti. 



From the observed facts it seems perfectly clear that there 

 exists in the cacti a mechanism for governing the rate of water 

 loss, which is entirely different in its response to external con- 

 ditions or in its daily periodicity from the corresponding mech- 

 anism in leafy plants. Just what the nature of either mechan- 

 ism may be cannot even be surmised as yet. 



Missouri Botanical Garden, 



St. Louis, Mo., March 9, 1907. 



COMPETITIOX BETWEEN TWO OAKS. 



By Dr. Roland M. Harper. 



The campus of the University of Alabama, near Tuscaloosa, 

 is shaded by many fine oaks of a decidedly southern type, i. e., 

 species with narrow entire glossy leaves, such as are never seen 

 in the cooler regions where most of the inhabitants of the East- 

 ern United States live. These oaks are of three species: Quer- 

 cus nigra L., the "water oak," Q. Phellos L., the "willow oak," 

 and Q. Immfolia Mx., which is variously known to the inhabi- 

 tants as water oak, willow oak or live oak, though all three of 

 these names belong more properly tO' other species. The persons 

 who planted these oaks, about the middle of the nineteenth cen- 



