232 



The increased richness of tlie milk in the dry season seems to 

 be recognized in all districts where the dry season is long enough 

 to permit the effect to become appreciated, but in localities where 

 the dry weather in which tapping can be done is short there is at 

 once less difference and less opportunity for it to become evident. 

 Where the dry season is long, as at La Zacualpa, the flow of milk 

 becomes small and tapping is deferred until some rain has fallen, 

 when the quantity and quality of the milk are both at their best. 

 The popular idea is that as the dry season advances the milk 

 becomes too thick to flow, and that during the rainy season it 

 becomes too poor in rubber to pay for tapping. The fact that the 

 latex becomes richer during the dry season does not prove, of 

 course, that the additional percentage of rubber in a measure of 

 protection against the dry weather. It may 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. 



I.ATEX IX 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 relatives of the Ceara rubber tree, ManiJwt 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 north-east of 

 Tehuantepec is another euphorbiaceous 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 Euphorbiace^, the most flourishing desert plants 

 were the Apocynacese, also with milky juice. The leguminous 

 plants of the desert do not contain latex, but they are noted for their 

 richness in gums and resins, which are similarly formed and may 

 have similar functions in the plant economy. 



The most striking suggestion of the utility of latex as a protec- 

 tion 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 Mammil- 

 larias 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 



