522 SECRETORY AND EXCRETORY SYSTEMS 



difficult to decide whether the foliar secretory organs, which are about 

 1 mm. in length, should be regarded as elongated sacs or as short 

 passages. All the features, accordingly, which have already been 

 considered in connection with internal glands, may recur in the case of 

 secretory passages. The glandular cavity is represented by a schizo- 

 genous, lysigenous or schizolysigenous passage. Schizogenous passages 

 are always lined by a sharply differentiated layer of secretory cells. 

 Very frequently, again, the whole passage is enveloped by a special 

 sheathing layer, which here assumes the characteristics of an endo- 

 dermis. 



The glandular cells of secretory passages are most frequently 

 elongated in the same sense as the duct itself : more rarely they are 

 transversely elongated (leaves of Cycadaceae). The relative size of the 

 secretory cells as seen in transverse section varies according to the 

 character of the surrounding tissues. If the duct is one which 

 traverses mesophyll {e.g. in Pinns, Abies, and other Conifers) or the 

 cortical parenchyma of a stem, the secretory cells appear comparatively 

 small. If, on the other hand, the passage is situated in the leptome 

 strand of a vascular bundle, the glandular cells appear relatively wide. 

 As a rule the walls of the secretory cells are thin and inclined to 

 protrude into the cavity of the duct. The walls of the cells lining 

 the mucilage-ducts of the Marattiaceae not only bulge inwards, but 

 are furnished with conical papillae ; in the foliar ducts of Lycopodiian 

 these papillae are represented by club-shaped outgrowths. In old 

 passages the secretory cells may even grow out after the manner of 

 tyloses and completely occlude the cavity. As in other glandular 

 structures, the secretory cells usually form a single layer ; in certain 

 cases, however {Hedera Helix, Philodendron), this layer undergoes one 

 or more tangential divisions at an early stage. 



The substances produced in secretory passages are very diverse in 

 character. In Lycopodium, among the Marattiaceae and Cycadaceae, 

 in species of Canna and Opuntia and in a few Araliaceae, a 

 mucilaginous or gummy material is produced in these organs ; among 

 Conifers, in certain Aroideae and Butomaceae, in the Alismaceae, 

 Umbelliferae, tubifloral Compositae and most Araliaceae, the secre- 

 tion consists of ethereal oil or resin, or of an emulsion of gum-resin 

 somewhat resembling latex in appearance. What has been said above 

 with regard to the process of secretion in the case of internal glands, 

 applies equally to secretory ducts. There is the same uncertainty as 

 to whether the secretion arises within the cavity of the secretory cell, 

 or in a " resinogenic " layer of the cell-wall. 



It has already been stated, that secretory passages are often 

 furnished with protective sheaths, which may be compared to those 



