102 CARNEGIE INSTITUTION OF WASHINGTON. 



The following theory has been formulated as a working hypothesis: 

 The increase or decrease of some substance or substances within the 

 tissues affects the water-attracting power of the colloidal jellies and 

 thus brings about a greater resistance to water-loss on the one hand 

 and a greater absorption by the roots on the other hand. This same 

 theory was found to be valid in the case of Opuntia versicolor, and plans 

 are laid for testing Encelia for the same phenomena. 



Streptanthus evidently has no successful means of coping with the 

 arid foresummer and so must complete its life-cycle within a month or 

 two, or in even less time. While Encelia lives in the open sunshine the 

 entire year, Streptanthus spends its short life in the partial shade of 

 larger shrubs. It has the ability of quick recovery from wilting as soon 

 as night conditions appear, but a more wilted condition each succeeding 

 week or day is soon followed by complete failure of recovery, and it is 

 seen that the plant has all the time been living under the most adverse 

 conditions it can endure. Its leaf water-content has a daily variation 

 of 5.7 to 2.7 grams of water per gram of dry weight. Although its 

 water-content is thus as high as that of Encelia for the same season, 

 and its leaf structure shows a much more mesophytic form, neverthe- 

 less its actual water-loss per unit area is only one-fourth as great as 

 from Encelia. The greater exposure of Encelia to the sun's rays 

 accounts for part of this difference, but probably there are other 

 agencies responsible also. 



As is the case with all desert annuals, the length of life of Amaran- 

 thus palmeri varies greatly with the length of the rainy season, but its 

 ability to revive after a shower when it has remained in a continuously 

 wilted state for three weeks makes it one of the most successful of the 

 annuals. Wilting seems to be its only means of cutting down trans- 

 pirational loss, for it loses four times as much water per unit area as 

 Streptanthus, while living under conditions of aridity which are three 

 times as great. It seems quite evident that Amaranthus owes its 

 ability to withstand the high evaporation-rates of August and Septem- 

 ber primarily to the efficiency of its water absorbing and conducting 

 systems and not to any agency which regulates transpirational loss, 

 such as appeared in the case of Encelia. The results of the experiments 

 show that Streptanthus could not endure the high evaporational rates 

 of August and September and that Amaranthus could thrive in the 

 spring were it not for the fact that its seeds will not germinate at the 

 lower temperatures which are characteristic of that season. 



