86 



CARNEGIE INSTITUTION OF WASHINGTON. 



the amount of water which they viay hold is greatly increased. This 

 results in giving the cell-mass a storage capacity which is permanent, 

 as the pentosans are not reconvertible into the polysaccharides. 



It is notable that in Castilleja, in which a detailed study of the 

 above facts was made, the acidity of the thin-leafed plants is about 

 double that of the succulents, which gives rise to the suggestion that 

 it is in plants with high acidity, or rather with a tjT^e of metabolism 

 which produces high acidity, that the development of succulence is 

 possible. 



That the change from the thin-leafed condition to that of succulence 

 is not a simple one and that the biocolloidal complex presents a dif- 

 ferent aspect is evidenced by the following hydration reactions of 

 both types of leaves in a fresh condition, after being swelled and dried, 

 and freshly dried. The figures relate to July 28-31, temperature 16° C. 



Hydration reactions of succvlent and thin leaves of Castilleja. 



Osmotic Concentration of Tissue Fluids in relation to Geographical Distribution, 



hy J. Arthur Harris. 



This work has been continued along the lines laid down in pre- 

 vious reports (Carnegie Inst. Wash. Year Book, 1915, p. 81; 1916, 

 p. 79, and 1917, pp. 88-89). 



A period of nearly a month was devoted to field work, with the 

 assistance of Mr. Charles W. Crane, in the pineland, hummocks, and 

 cypress swamps of the Lake Okeechobee region of southern Florida. 

 Three periods of work, aggregating several weeks in duration, were 

 devoted to the vegetation of the Dismal Swamp, with the cooperation 

 of Dr. John Y. Pennypacker. Several hundred cryoscopic determi- 

 nations were obtained in these highly interesting regions. Discus- 

 sions of two phases of the work have been published during the year. 



In a memoir on the vegetation of the Jamaican Coastal Deserts, 

 published with Mr. John V. Lawrence (Bot. Gaz., vol. lxiv, pp. 285— 

 305), the conclusions concerning the high osmotic concentration of 

 the tissue fluids of desert plants based on earlier studies on the Ari- 

 zona deserts (Phys. Res., vol. ii, pp. 1-50, 1916) have been fully con- 



