DERMAL GLANDS 



,513 



chloroplasts and constitute the photosyntbetic apparatus of the 

 gland. 250 " 



The process of secretion is not precisely the same in all dermal 

 glands. The secretion of a mucilage gland, for instance, is from its 

 first origin a constituent of the cell-membrane and not of the contents ; 

 certain sub-cuticular layers of the wall become mucilaginous, with the 

 result that the cuticle is distended in a vesicular manner and finally 

 ruptured. 



In the case of glands which secrete ethereal oil or resin, the 

 secretion likewise generally originates within the cell-wall ; this view 

 was first put forward as a suggestion by Hanstein, 

 and was definitely enunciated by De Bary. After 

 the cell-wall has become considerably thickened, 

 the secretion appears in constantly increasing 

 quantity between the cuticle and the cellulose layers. 

 This deposition of secretion in the substance of the 

 cell-wall sometimes begins, first of all, near the apex 

 of the glandular head ; in other instances it com- 

 mences simultaneously at several points (Fig. 209). 

 The vesicular distension of the cuticle, which takes 

 place in this case also, may ultimately affect the 

 walls of adjacent non-secretory cells, if the quantity 

 of secretion produced is large ; according to Hanstein 

 this distension is facilitated by a previous gela- 

 tinisation of the sub-cuticular layers of the wall. 

 As a result a glandular cavity is formed, in which 

 the secretion accumulates. The cuticle is incapable 

 of indefinite distension, and finally breaks, whereupon the secretion 

 escapes to the outside. Hanstein further states that this cuticle may 

 be regenerated in certain cases ; when this occurs, the whole secretory 

 process may be repeated several times. 201 



More rarely the secretion, instead of being deposited in the outer 

 walls of the glandular cells, appears within the partitions that separate 

 adjacent secretory elements. This intramural type of gland is 

 exemplified by the orbicular scales which cover the lower surface of the 

 leaves of many species of Rhododendron and by the embedded multi- 

 cellular foliar glands of Psoralea (cf. above, p. 511). 



Complete uncertainty prevails with regard to the actual manner of 

 formation of the secretion in the substance of the cell-wall, in all these 

 cases. It is conceivable that the secretion owes its origin to a chemical 

 metamorphosis of certain layers of the membrane, which are regenerated 

 as fast as they are transformed. But it seems, on the whole, more 

 probable that the raw material employed in the manufacture of the 



2k 



Fig. 209. 



Glandular hair of Pelar- 

 gonium zonale, plasmo- 

 lysed and treated with 

 alcohol. 



