233 



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 watery 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. 



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 ia all plants possess- 

 ing them to perform a cnmmon function, then 1 am inclined to think the idea of 

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

 Laticiferous plants are markedly characteristic of tropical regions, where transpira- 

 tion is great. The development of a system of tubes running throughout the plant 

 to be tilled 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 

 " Vegetation of Tropical America," mentions lianas and other plants of tropical 

 forest and scrub as often laticiferous, and says : " Most likely latex serves several 

 purpnses, 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 < f 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 surround- 

 ing tissues. The genus Euphorbia chiefly inhabits dry regions and ia one of the 

 richest in latex. 



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

 is one to be borne in mind in studying ihe role of latex in plants, and hitherto it 

 has in the main been disregarded. If latex does serve as a water reserve, thi n 

 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 aniiount 

 of water actually stored or present at one time in a tree like 

 Castilloa 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 transpira- 

 tion, but of these alternatives the facts seem to be much more in 

 favour 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 



* Parkin, Ann. Bot., 14: 212-21l^ X900, 



