38 CENTRAL AMERICAN RUBBER TREE. 



WATER STORING AS A FUNCTION OF LATEX. 



As already stated, the recognition of a relation between latex and 

 dry weather has been hindered rather than helped by the attempt at 

 framing a theory of the use of latex to the plant; but a few writers 

 have appreciated such facts as the above, and have been inclined to 

 look upon the storage of water as the long-sought general function. 

 The following extract affords an instance: 



If the formation of laticiferous tubes has been called forth in all plants possessing 

 them to perform a common function, then I am inclined to think the idea of their 

 serving as channels for holding water in reserve as one of the most plausible. Lati- 

 ciferous plants are markedly characteristic of tropical regions, where transpiration is 

 great. The development of a system of tubes running throughout the plant to be 

 rilled with water during the wet season and then to be gradually drawn upon during 

 times of drought is intelligible. 



Warming, in a paper in the Botanical Gazette for January, 1899, entitled "Vegeta- 

 tion of tropical America," mentions lianas and other plants of tropical forest and 

 scrub as often lacticiferous, and says: "Most likely latex serves several purposes, 

 and one of them, I suppose, is to supply water to the leaves in time of need when 

 transpiration becomes too profuse." 



From our experiments in Ceylon we found that the quantity of latex extractable 

 from incisions in the trunks of Hevea trees varied considerably with the time of the 

 year and seemed to depend largely upon the available moisture in the soil. After 

 heavy rain the exudation of latex is much more copious and thinner, looking as 

 though the vessels had become surcharged with water. 



As the necessity for a reserve of water increased, the laticiferous system would 

 tend to become more extensive and more intimately associated with the surrounding 

 tissues. The genus Euphorbia chiefly inhabits dry regions and is one of the richest 

 in latex. 



This view does not explain the proteid or starch grains of latex, yet I think it is 

 one to be borne in mind in studying the role of latex in plants, and hitherto it has 

 in the main been disregarded. If latex does serve as a water reserve, then perhaps 

 it is chiefly valuable for the growing organs." 



This view has, however, met with no general acceptance, and has 

 obvious difficulties, the most important being that the amount of water 

 actually stored or present at one time in a tree like Castilla would not 

 long suffice for necessary transpiration. It avails little for such a 

 plant to store unless it is also possible to husband the supply. At 

 present, however, there seems to be no practical suggestion of means 

 by which latex rich in rubber could better assist either in storing the 

 water or in preventing transpiration, but of these alternatives the 

 facts seem to be much more in favor of the latter. Apart from the 

 slight increase due to growth, the contents of the trunk must remain 

 of approximately the same volume. The increased pressure to which 

 is due the increased flow of milk after the rains begin does not require 

 a large increase of the volume of liquid in the tree, and is in all prob- 

 ability greatly assisted by its greater fluidity, which enables it to flow 

 longer distances to the cuts, the capillary friction being decreased. 



a Parkin, Ann. Bat., 14:212-218, 1900. 



