15 



gave shade in the morning and afternoon, and the atmosphere was 

 undoubtedly kept far more humid throughout the day than would 

 be the case in a large tract of unshaded land baked by the tropical 

 sun. They were also undoubtedly assisted by a mulch of dead 

 leaves and brush. Trees 12 feet high were said to be only I 

 year old. 



It would seem, then, that one of the extreme suggestions — the 

 planting of rubber in the undisturbed forest — is clearly inadvisable 

 and may be dismissed from further consideration. The other 

 extreme — clean culture — is not so readily condemned as impracti- 

 cable, since observations in southern Mexico establish the fact that 

 even single trees, standing in the open sun and with little other 

 vegetation near them, are not only able to survive six months 

 of dry weather, but actually remain more leafy at the end of the 

 dry season and thus appear to suffer less from drought than those 

 on land covered with weeds and bushes. The reason for this 

 apparent anomaly may not be difficult to conjecture, since it is 

 plain that a tree standing in cleared ground has a monopoly of all 

 the moisture which rises in the soil, and may thus have a distinct 

 advantage over one obliged to share a similar supply of water 

 with a tangled mass of other plants which expose to the atmosphere 

 a total leaf surface many times that of the young rubber tree. 

 Moreover, it is also clear that the water required to supply the 

 needs of this large amount of vegetation would greatly exceed 

 that which escaped from the exposed surface of the soil. It is 

 even doubtful whether a covering of low vegetation greatly checks 

 the evaporation from the soil ; it may be as great or greater 

 than where the surface of exposed soil is loosened by stirring and 

 thus fornjs a layer which hinders the access of dry air and is a 

 nonconductor of heat. In previous discussions of shade in the 

 culture of Castilloa this distinction between open culture and clean 

 culture seems to have been overlooked, and the question of shade 

 has continued to be confused with that of water supply. The 

 statements of various writers that the leaves are unable to withstand 

 exposure to the full sun because of their delicate texture are quite 

 erroneous. The tree needs sunlight, and it is benefited by it as 

 long as the water supply is sufficient, but when this becomes de- 

 ficient the leaves shrivel. The light is no brighter and the tem- 

 perature no higher in the dry season, which in Mexico occurs in 

 the winter months ; but the dry atmosphere demands more water, 

 while the soil supplies less. 



The rapidity with which dry atmosphere takes water from a 

 plant may be judged by the promptness with which the leaves of a 

 broken branch will and shrivel, and this happens very promptly with 

 Castilloa. Many plants have developed no expedients for resist- 

 ing evaporation and are accordingly confined to continuously 

 humid regions, but Castilloa, as has already been seen, is adapted 

 in several ways for resisting drought. The leaves themselves are, 

 it is true, of rather loose texture and have only the slight assistance 



