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ration, and by means of this and an early leaf-fall it could survivv 

 the vicissitudes of the drought. — »-■ - 



I will also briefly relate one of my laboratory experiments, 

 which has a bearing on this matter. Two Cast ilia seedlings were 

 grown in pots, and when they were four inches high one was 

 placed under a double glass bell, the outer room of this filled with 

 an orange-coloured liquid to give the desired light, and the inner 

 bell constantly filled with well saturated air. The other seedling 

 was placed in a bottomless glass cylinder, and by means of a fan, 

 kept going by a clock movement, a constant exchange of air was 

 secured in the cylinder. This was further placed so that it was 

 exposed to the sun all day long. The roots received all the water 

 they could absorb, and thus the transpiration was kept at a 

 maximum. First the latter plant was very weak, but gradually 

 recovered strength. After three weeks both plants were examined, 

 both micro- and macroscopically as to latex. The plant in the 

 moist air had well developed latex vessels, but the liquid in 

 these was thin and without any formation of globules. The plant 

 which had been exposed to excessive transpiration had the 

 ordinary latex of young seedlings, but rather concentrated. To 

 make sure that this result was not merely caused by an individual 

 or inherent character of the seedlings employed, I renewed the 

 experiment, taking care to select seedlings which did not appre- 

 ciably differ in any respect. The result was again the same. 



I consider that this shows that when Castillo, is grown under 

 certain conditions the quantity of latex produced in the tree is 

 reduced to a minimum, while under conditions favourable to or 

 assisting excessive transpiration Castilla will produce latex as a 

 means of protection. 



How are we to explain the fact that Castilla in some places in 

 the mountains of Southern Mexico, where the rainfall is high and 

 the atmosphere laden with moisture, does not produce latex, or at 

 least a very small quantity ? I take it to signify that in those 

 places the transpiration is less than on the plains. This is, of 

 course, only assumption, and the question is still open. 



It has been claimed by certain theorists that the sole or at 

 least the principal function of latex is protection against the 

 natural enemies of the plants. How is it then that those individual 

 trees of the same species, which do not produce latex, are not 

 instantly devoured by insects and other pests ? Let those who 

 know answer this. I think that the conclusion of latex being 

 solely a protection against insects or other animals is as narrow 

 as the theory that spines are developed on certain plants for the 

 same purpose, solely for protection against animals. No plant 

 physiologist or ecologist who has studied the question in nature 

 would now-a-days maintain this. I do not deny that latex in the 

 rubber trees may incidentally be a means of protection also against 

 insects, but this certainly is not its main function. 



