MILK OF DESEKT PLANTS. 37 



rubber is a measure of protection against the dry weather. It ma}*' 

 be that the rapid growth which goes on in the rainy season uses up the 

 rubber, while the cessation of growth in the dry season permits it to 

 accumulate. This possibility does not, however, exclude the other, 

 but seems rather to strengthen it, since there are other reasons for 

 believing that the possession of latex is an advantage in the struggle 

 against drought. Several such facts were noticed during a recent 

 visit to southern Mexico. 



LATEX IN DESERT PLANTS. 



The plants able to make the most vigorous growth and put out flowers 

 and new leaves at the end of the dry season, even in the cactus deserts 

 about Tehuantepec. belong to the genus Jatropha and are near rela- 

 tives of the Ceara rubber tree, Manihot glaziovii. Also Prof. H. 

 Pittier says that on the dry Pacific slope of Costa Rica the Ceara rubber 

 tree produces rubber, but refuses to do so in the humid district of 

 Turrialba, although it thrives well there. 



In the cactus desert about San Geronimo to the northeast of Tehuan- 

 tepec is another euphorbiaeeous plant with naked green stems a yard 

 or more in length and reddish unsymmetrical flowers. The stems are 

 rich in a milky juice, which rapidly coagulates into a substance much 

 like rubber, but lacking elasticity. The plant was quite leafless, but 

 was blossoming at the end of the dry season. After the milky Euphor- 

 biacese, the most flourishing desert plants were the Apocynacea?, also 

 with milky juice. The leguminous plants of the desert do not have 

 latex, but the} T are noted for their richness in gums and resins, which 

 are similarly formed and may have similar functions in the plant 

 economj'. 



The most striking suggestion of the utility of latex as a protection 

 against drought was noticed in a cactus of the genus Mammillaria, 

 found nestling in the crevices of the bare, black rocks of the fiercely 

 heated hillsides about Tehuantepec. The Mammillarias differ from 

 all other members of the family in having a thick, milky juice, which 

 becomes very sticky between the fingers, though showing no signs 

 of elasticity. It will be difficult to avoid the conclusion that in this 

 instance the milky juice is the special character which has enabled the 

 Mammillaria to excel all its relatives in resistance to desert conditions 

 of extreme heat and dryness. 



A step in the same direction seems also to have been taken by a 

 large, straggling Opuntia found near San Geronimo. Instead of the 

 wateiy juice found elsewhere in this genus, a knife cut brings out a 

 thickish, opalescent sap, which rapidly coagulates into a somewhat 

 resinous substance and quickly seals over the injury. 



