PITTIER — TREATMENT OF THE GENUS CASTILLA. 251 



detached from the original C, elastica. Mr. G. Js. Collins not only 

 collaborated with Mr. Cook in all his researches, but contributed on 

 several other occasions to the increase of the alcoholic collection of 

 specimens. The present writer had also his modest share of the 

 field work. Mention should also be made here of a fine set of nat- 

 ural-size photographs, made mainly by Mr. C. B. Doyle under the 

 direction of Mr. Cook. This collection is unique of its kind and 

 affords ample and conclusive evidence bearing upon the identifica- 

 tion of the several species. 



These are the antecedents that have led the writer to undertake 

 the present monograph of the genus Castilla, as a preliminary step 

 to the study of questions relating to the availability of the species as 

 rubber producers. The detailed investigation of the above-mentioned 

 material not only established the value of Mr. Cook's species, but led 

 to the segregation of an additional Central American form. Besides 

 these, a new species Avas discovered by the writer in Colombia, and 

 the well-known German explorer, Doctor Ule, identified as a Castilla 

 one of the Brazilian rubber trees, the number of species described in 

 the present paper thus becoming ten. On the oilier hand, it has not 

 been possible to take into serious consideration certain proposed new 

 species, as, for instance, C. alba, ('. nigra, and C. r}ibra of Koschny, 

 that are mere physiological accidents, the results mostly of differences 

 in light and soil conditions. This has been clearly shown by Mr. 

 Cook's investigations in Costa Rica and by careful examination of 

 the specimens. 



GENERAL CHARACTERS AND THEIR SYSTEMATIC VALUE. 



HABIT. 



When fully developed, all species of Castilla are trees of large size 

 and of striking habit. As noticed by Mr. Cook, they are seldom true 

 forest trees, but they also avoid the open grass-covered savanna. 

 Their natural habitat is in the clearings and other open spots of the 

 virgin forest, where they are generally found in company with 

 Cecropia, or in the fertile, sparsely wooded alluvial Hats of the valley 

 bottoms. 



HOOTS. 



In these rubber trees the root system consists of a rather short tap 

 root and of several lateral roots spreading horizontally and so near 

 the surface of the soil that they can often be followed for 20 to 30 

 meters. This disposition is very adverse to the cultivation of the tree 

 in pastures, because the treading of the cattle often damages the trees 

 beyond recovery. In the flat, shady forest the roots often assume a 

 buttress-like shape at their emergence from the main axis. These 



