>o8 



INTERCELLULAR SPACES. 



in the epidermal layer; each of these is again divided into an inner cell next the 

 parenchyma, and a superficial cell (d, c). The superficial cells increase further so as to form 

 the portion of the epidermis which covers the cavity (B, c, d). The inner ones take a 

 direct part in the formation of the cavities. The chief part of the cavity originates it is 

 true from the products of division of the primary parenchymatous cell {A, p, />), which 

 divides by alternate horizontal and vertical divisions into numerous daughter cells ; these, 

 together with the similarly formed products of division of the inner epidermal cell, form 

 a compact round body of numerous small cells. In the protoplasm of all cells of this 

 body, which are at first very granular, there appear, after the division and growth have 



ended, more and more numerous drops of ethereal oil ; 

 then the delicate membranes are dissolved, and the oil 

 drops coalesce to large drops (o in C). The process 

 begins in the middle of the body, and proceeds cen- 

 trifugally to its surface. The oil-containing cavity 

 thus formed is limited, with the exception of the 

 epidermis, by cells of the surrounding parenchyma, 

 which are more or less flattened towards the surface 

 of the cavity ; these cells, being in uninterrupted con- 

 tact with one another, completely enclose the latter. 

 The cavity in the hairy dermal warts of the Dittany 

 arises, as described p. 69, in the same way. 



The oil-cavities of Ruta have in the main the same 

 development. That of Citrus differs at most in small 

 secondary peculiarities, which need not be described 

 in detail here. iNIartinet, who described them, has traced 

 their origin back to a condition in which the transverse 

 section shows three small cells in the epidermal layer, 

 with abundant protoplasm, and beneath these three 

 inner cells. The arrangement of these cells is such, in 

 these youngest cells and in rather later stages (A c, 

 Fig. 234 and 235), that it is probable that their mode 

 of origin is the same as that described by Rauter for 

 Dictamnus. It can hardly be doubted, after investiga- 

 tion of mature or half-formed stages, that those of 

 the other members of the group of Rutaceae have 

 fundamentally the same origin. 



The cavity is always completely and smoothly iso- 

 lated by the close connection of the cells of the sur- 

 rounding tissue, so that in good sections it may be 

 taken for a single large cell lying between these. As is especially evident in the chloro- 

 phyll-containing parenchyma, the cells of the limiting layer do not differ fundamentally 

 in structure from those of the mass of tissue, in which the cavity lies. Obvious remnants 

 of partially dissolved delicate cell-membranes are not unfrequently to be found ; these 

 form a more or less irregular covering to the wall. It is possible that in many cases 

 the mass of delicate cells, instead of being dissolved, persists in whole or in part. Many 

 figures of Engler appear to point to this. But here, even with tolerably good pre- 

 parations, it is possible to be deceived, since the sections often do not cut the cavity in 

 a median plane, but lay bare portions of their wall, which then appear in surface-view as 

 dense, multicellular bodies. 



{c) The spots, recognisable with the naked eye as transparent points, in the lamina of 

 Hypericum perforatum, and its allies, are oil-cavities of flattened spherical form, which 

 occupy almost the whole space between the portions of epidermis of both leaf-surfaces 

 which cover them, and are separated from the lower epidermis by at most one layer of 

 parenchyma. Their structure, or that of the tissue surrounding them, is fundamentally 



Fig. 86. — Dictamn-js Fraxinella ; oil-resenoirs 

 of the upper side of the leaf; transverse sec- 

 tion ; C (200) mature ; A and B successive yoiing^ 

 staj^es of development (320). Further description 

 in text After Rauter from Sachs' Textbook. 



