20 



such hairs occur the trees were below the average in regard to 

 yielding capacity. It seems therefore obvious that hairs should be 

 absent in the future type of rubber trees. There is also another 

 reason for this. We know that the hairs are one mode of protecting 

 the leaves against excessive transpiration. The latex is another 

 means by which the tree prevents its water supply from evaporating 

 too rapidly through the leaves. If we develop a tree without hairs 

 we should be able to force the tree into preparing more latex in 

 order to keep up the equilibrium, not allowing too much water to 

 transpire. 



Remarks on Function of Latex. 



By the above I do not mean to say or indicate that 1 consider 

 the function of latex as solely one of water storage or prevention of 

 too rapid evaporation. But field observations as well as laboratory 

 and breeding experiments have conclusively shown that the protec- 

 tion of the plant against too rapid transpiration is one of the 

 functions of latex, at least in Castilla. 



I could give a number of proofs for this, but as the question is 

 more fully discussed in my Handbook on Castilla, above referred to, 

 I will here mention only one instance, which first fell under my 

 observation in July 1905. In walking through a stand of four year 

 old rubber trees, one early morning, I stopped and measured some 

 two dozen trees, which were especially well developed. I noted 

 down in my field-book certain characteristics of these trees, and 

 with my thermometers, took the soil and atmospheric tempera- 

 tures, near one of the trees standing about in the middle of the 

 group of trees measured. I intended to return two hours later, when 

 the full heat of the sun had been acting on the trees to re-measure 

 the trunks in order to find out the shrinkage of the trunks at 

 different times of the day. I was delayed, and returned some four 

 hours later instead, at 10-47 ^•'^- ^ ^^^ ^"^^^ thing I noticed was 

 that one of the trees which in no way differed from its neighbours 

 upon my first visit, was now looking very exhausted, with drooping 

 branches and leaves hanging limply downwards. Another tree 

 standing by looked perfectly fresh, while some of the others showed 

 signs of having been slightly affected by the now scorching rays of 

 the sun. This difference in power of withstanding sudden high 

 temperature was so marked that I decided to try the different trees 

 for latex. The result of a small cut in each tree was that A, the 

 tree with drooping leaves, had no latex, while H, the most fresh 

 looking tree of the lot, had plenty. Between these B, C, etc., 

 showed a gradual increase in the amount of latex with the exception 

 of two trees F and G, of which the latter had less latex than the 

 former, but it was much thicker. This year I noticed the same 

 difference at the end of January. Closer observation now revealed 

 the fact that A had much more hairs on the leaves, petioles, and 

 branchlets than H, and thus ought to have been better protected 

 against too rapid transpiration. Counting the stomata on the 

 leaves, I found that the number was smaller in A, — another 

 protective device. The barkpores were almost equal. Five weeks 

 later A commenced to drop its leaves, and was almost bare at the 

 end of March, when H still had all its foliage left. A did not 



