INTERNAL GLANDS 



51 U 



portant of all is the character of the lateral or pore-walls of the cover- 

 cells. These walls contain a middle lamella of varying thickness, which, 

 to judge by its staining properties, consists largely of pectic compounds, 

 though in Ruta and Pilocarpus some callose is also present. The pore 

 arises through the splitting of the walls along this relatively incoherent 

 layer. When this process of separation which is more fully described 



Fie. 211. 



Discharging mechanism of the internal glands of Ruta graveolcns. A. Cover and pore 

 in surface view. B. Cover and pore in vertical section. 



below is completed, the cross-sectional outline of the cover often 

 strikingly resembles that of a stoma.' 2 ' 57 



The one- to three-layered " body " of the gland, which represents 

 the active portion of the discharging apparatus, consists of flattened 

 cells arranged in a perfectly continuous layer. Its principal duty is to 

 exert pressure upon the contents of the gland by means of the turgor 

 developed in its constituent cells. The presence of a high internal tension 

 in these cells may be readily demonstrated, if a moderately thick 

 transverse section of a living leaf of Ruta graveolens which must, of 

 course, traverse a gland is examined in water. Under these conditions 

 the inner walls of the uninjured body-cells protrude to an extraordinary 

 extent into the glandular cavity ; the cells in question then resemble 

 large vesicles, whereas in the intact gland they are quite Hat. The 

 extension of the inner walls of the body-cells [in water] may amount 

 to between 25 per cent, and 80 per cent., and greatly exceeds their 



