518 SECRETORY AND EXCRETORY SYSTEMS 



by their form and contents, and often also by their smaller size. In 

 shape these secretory cells are either approximately isodiametric or else 

 tangentially flattened ; in the former case, the " free " walls usually bulge 

 more or less into the glandular cavity, and may actually be developed as 

 protruding papillae {Hypericum perforatum). In certain Papilionaceae 

 {Lonchocaipus, Derris, Milletia) the secretory cells are tubular and 

 tangentially curved. The contents of the cells usually consist of 

 colourless granular cytoplasm, enclosing a nucleus which is often rela- 

 tively large. Opinions differ as to the mode of formation of the secreted 

 substances and as to the manner in which they escape. Many observers 

 assume that the ethereal oil or resin which constitutes the secretion, arises 

 first of all in the interior of the glandular cells, and subsequently passes 

 through the cell-wall. Tschirch, on the contrary, maintains that the 

 secretion is produced within the cell-wall from a special mucilaginous 

 stratum or " resinogenic layer " ; according to this view, the protoplasts 

 of the glandular cells never contain any of the actual secretion, but 

 merely supply the " resinogenic " material. 



In the majority of plants with internal glands, the secretion is 

 never actually expelled. A gland may therefore also act as a secretory 

 reservoir ; this is, in fact, its only function where the glandular cells 

 become inactive through age and finally die, or where the cavity 

 develops lysigenously and the secretory cells consequently disintegrate 

 at an early stage. In a few families, however, special arrangements 

 are found which enable the contents of the glandular cavity to escape. 

 A special discharging mechanism was first described by the author in 

 connection with the sub-epidermal glands of certain Rutaceae. Here 

 the gland is made up of a passive portion or " cover," and an active 

 part or " body." The cover generally consists of four cover-cells, which 

 arise by the division of a protodermal mother-cell (Fig. 211 a). The 

 shape, structure and chemical composition of the membrane of the 

 cover-cells is such that a glandular orifice arises in a predetermined 

 region of the lateral walls (which may accordingly be termed the " pore- 

 walls "). The cover-cells are almost always considerably flattened verti- 

 cally, in comparison with the adjoining epidermal elements ; in Ruta 

 graveolens and Pilocarpus pinnatifolius they are sunk below the level 

 of the general epidermis (Fig. 211 b). This flattening of the cover- 

 cells facilitates the formation of the pore. It is also an advantage that 

 the outer walls of the cover-cells or, at any rate, their cutinised layers 

 are, as a rule, appreciably thinner than the corresponding portions of 

 the neighbouring epidermal cells. In Agathosma piibescens the cutinised 

 layers of the cover-cells are, indeed, no thinner than usual ; but they 

 are traversed by a deep furrow, which follows the line of the pore-wall 

 and thus prepares the way for the formation of the pore. Most im- 



