INTERNAL GLANDS 



517 



fragments 



of their cell-walls as may have escaped 



from such 

 destruction. 



It should, however, be noted that in many cases the glandular 

 cavity begins its development in a schizogenous manner, but later 

 on undergoes lysigenous enlargement. The author was the first to draw 

 attention to this composite mode of origin of glandular cavities, which 

 was discovered by him in the leaves of Ruta graveolcns (Fig. 210 c); 



Fio. 210. 



Internal glands of various foliage-leaves. A, Hypericum. perforatum. The sheath of 

 the gland (/*) consists of thin-walled tangentially-flattened cells; the secretory cells 

 (s) are more or less papillose. B. Myrtut zeylanica. The secretory cells are flattened 

 and have thick outer walls. C. Jiuta graveolcns (young gland); h, sheathing layer. 

 The glandular cavity is partially occluded by secretory cells which have become 

 detached from the wall. D. Dictamnus albus. 



it is, however, by no means a rare phenomenon, a fact which probably 

 accounts for the frequent lack of agreement between different observers 

 who have described the development of internal glands. Tschirch 

 subsequently applied the term schizolysigenous to glands which have 

 this composite origin. It must also not be forgotten, that the growth of 

 the organ as a whole is liable to bring about a considerable secondary 

 enlargement of glandular cavities, whether their early development is 

 schizogenous or lysigenous in character. 



The secretory tissue of a schizogenous gland generally consists of 

 a single layer of glandular cells (Fig. 210 a, b), which are almost 

 always readily distinguishable from the cells of the adjoining tissues 



