CELLS WITH SPECIFIC FUNCTIONS. 



79 



Each gland is made up of the following parts (see Fig. 62) : — 



(a) An external layer of rather thiok-walled cells, arranged con- 

 centrically. 



(/3) An internal layer of thin-walled cells full of granular contents. 



This is the so-called endothelial layer. 



(y) A central cavity, in which may be seen globules of oil and a 

 small quantity of cell-debris. 



The cells of the endothelial secretory layer break down and 

 disorganise, thus setting free the oily secretion. Mixed with 

 this oil is a certain amount of cell-sap, which confers considerable 



Fig. 62.— A Section across an Oil-gland in the Outer Cortical 

 Tissue (pericarp) or Gitrus Aurantii. — e, Epidermis ; c, cortex ; 

 ep, endothelial secreting layer; g, oil-globules lying in the central 

 cavity ; p, thick-walled cells just outside the gland. 



turgidity upon the gland. At times the gland may burst through 

 the cortex and epidermis, setting free the secretion on the 

 external surface. 



B. Resin-canals. 



These structures occur typically in the cortex and xylem of 

 stem and root of Pinus, and also in the leaves, where they are 

 surrounded by the cells of the spongy parenchyma. 



In a transverse section of the stem or leaf each resin-canal 

 is seen to possess the following parts (see Figs. 63, 64, 65, 65a) : — 



1. An outer layer of thiok-walled elements one or two deep. This layer 

 forms the guard-ring of the canal. 



