MIXED CEOP OP USIFOTIM AGE. 71 



covering tissue, which becomes green under exposure to light. 

 Not uufrequently the entire cambium layer, the medullary rays ex- 

 cepted, may dry up, when the same covering of healing tissue 

 may be formed exclusively by those rays. However it be, the new 

 tissue soon differentiates itself into wood, cambium and bast, with 

 a corky layer outside of all. Now it is evident that the chances 

 of the cambium remaining moist sufficiently long for the forma- 

 tion of this protective covering will depend not only on the con- 

 dition of the weather (rain, a saturated atmosphere, &c.), but also 

 on the secretion of gum or resin, as in Acacia arabica, A. Catechu, 

 Boswellia serrata, Pinus lonyifolia, &c., or on a continuous and 

 abundant outflow of sap, as when a tree is coming into new leaf 

 (sal, mahua, &c., in the hot w r eather), and so on. 



In the third alternative, the wound can be covered only by 

 means of the gradual closing in of the edges by the abnormally 

 active formation of new, constantly advancing tissue there. Now 

 we know that the rapidity of this kind of healing depends to such 

 an extent on the thinness and vitality of the new bark formed 

 along the encroaching edges and on its comparative freedom from 

 an external layer of hard inelastic dead bark, that the largest 

 wounds may become completely closed over in a short time in 

 favoured species, while even alsolutely small wounds may remain 

 open for a long series of years in less fortunate species. Thus is 

 accounted for the great ease with which such wounds heal up in the 

 Boswellia serrata^ in teak, &c. 



XIV. RELATIVE FACILITY OF RENEWING LOST OR DAMAGED 

 AERIAL ORGANS. The facility with which a lost or damaged organ 

 may be renewed will depend on one or more of several conditions, 

 the principal of which are 



(a) Ability to develop adventitious buds. At all ages such buds 

 may, more or less easily and in greater or less abundance according 

 to the different species producing them, form on the section where 

 the organ has broken off or has otherwise been injured. In very 

 young individuals of most of our broad-leaved species, the presence 

 of collum buds is the surest guarantee of their ability to survive 

 almost any degree of mutilation. As a rule, those species 

 produce most collum buds, which, as young seedlings, develop a 

 stout taproot compared with the small and, for their age, generally 

 insignificant-sized aerial portion ; such are sal, teak, Sterculia, 

 Dalbergia lot (folia and Sissoo, &c. And in fact those individuals 

 are most prolific in such buds which, within certain limits, are tho 

 least vigorous, the otherwise arrested energy of the young plants 



