Vol.6 



No. 2 



TORREYA 



February, 1906 



THE EFFECTS 



OF HIGH RELATIVE HUMIDITY 

 ON PLANTS 



By W. a. Cannon 



The water relations of the desert plants are so dehcately ad- 

 justed that an apparently slight variation in the available supply, 

 either an increase or a decrease, or, what is directly associated 

 with the water relations, in the humidity of the air, produces an 

 instant and notable effect. This is not new, as a matter of fact, 

 but new instances of it may be worth recording. I have already 

 shown * how readily the ocotillo {Fonqiiicria splcndens) responds 

 to an increase in the water supply. Up to June 29, 1904, for 

 several weeks the particular ocotillo referred to above was with- 

 out leaves. This condition was directly connected with a long 

 period of drought. On June 29, three gallons of water were 

 poured slowly on the ground at the base of the plant; on July 

 I, leaf-buds were seen, which at 2 p. m. on the following day 

 had become i cm. long, and four days afterwards the leaves were 

 fully grown. But owing to the continuation of the drought, 

 and the small amount of moisture in the air the leaves thus 

 artificially produced very early withered and fell away and after 

 about a fortnight the shrub was again denuded. The response 

 of ocotillo to an increase in the water supply was observed on 

 other occasions which are narrated in the paper above cited. 

 Similar reactions have been seen in Encelia farinosa, Cereiis gi- 

 ganteiis, Parkiiisonia micropliylla and in other plants. 



From what has already been said, it is apparent that a want or 



a supply of sufficient rain must materially modify the activities of 



the desert plants. For example, in the summer of 1904 the 



■_ rainfall was nearly normal and the desert abounded in a pro- 



''''\:x^xi%^\x^^^.oxi. oi Fouquitria splendens. Bull. Torrey Club 32 : 41 1. 1905. 

 "^ [No. I, Vol. 6, of ToRREYA, comprising pages 1-20, was issued January 25* 



NEW YORK 

 80TAN1C; 



1906.] 



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