132 



BOTANY 



PAKT I 



commonly happens in the Oak, when, after the destruction of leaves by caterpillars, 

 a second formation of spring wood is occasioned by the new outgrowths thus 

 induced. In the wood of tropical plants the annual rings'may be entirely absent. 

 This occurs, for example, in the tropical Conifers of the genus Araucaria, which, 

 in this respect, show a marked contrast to the Conifers of the northern zone. Any 

 interruption of growth, such as would occur during a drought, followed by a 

 period of renewed activity, may occasion the formation of annual rings even in 

 tropical plants. 



Although a cessation in the formation of wood takes place so 



Kio. 141. A, Transverse section of the wood of a Pineat'the junction of two annual linus. ./. Earl\ 

 wood ; s, late wood ; t, bordered pit ; u, interposition of a new row of tracheides resulting from 

 the radial division of a cambium cell ; A, resin canals; m, medullary vays.; </, limit of lat>- 

 wood, (x 240.) If, Part of a transverse section of the stem of a Pine, s, Late wood; c, 

 cambium ; v, sieve-tubes ; p, bast parenchyma ; A-, cell.of bast parenchyma containing crystal : 

 cv, sieve-tubes, compressed and functionless ; HI, medullary ray. (x 240.) 



early, the cambium tissue continues to form bast so long as climatic 

 conditions permit. As a rule, however, far'fewer elements are added 

 to the bast than to the wood. Owing to the continued growth in 

 length, the higher a cross-section is made of a gymnospermous or 

 dicotyledonous stem the fewer will be the annual rings. As the 

 diagram in Fig. 140 shows, the older rings disappear first on passing 

 towards the apex, and in the same way the older layers of the bast 

 are unrepresented nearer the apex. Up to a certain period, in the 

 age of woody plants, the elements of both wood and bast exhibit a 

 progressive increase in size. 



The living elements may remain in a state of greater or less 



