THE METABOLISM OF PLANTS. 



245 



Fig. 31 (after Kreuz). Section of a resin-duct containing resin (ti) from 

 a leaf of Pinus sylvestris. 



cellulose of the cell-wall ; it is probably derived from the 

 protoplasm, and is merely deposited in the cell-wall. The 

 actual excretion is usually effected, in the case of superficial 

 glands, by the rupture of the cuticle which is continuous 

 over the gland, and the consequent escape of the contents 

 (Fig. 32) : in some cases the gland remains closed, and any 

 volatile substances (ethereal oils) which may be present 

 escape by evaporation. 



fe 



32. (after de Bary.) Glandular hairs of Primula sinensis. The secretion 

 collects between the cuticle and the deeper layer of the cell-wall. In a the 

 accumulation of the secretion is commencing : in b it has become consider- 

 able : in c the cuticle has ruptured, allowing the secretion to escape. 



