484 PHYSIOLOGY. 



Gelatinous Layers, Besides the primary membrane and the 

 secondary formations, we find in certain cases a kind of envelope 

 which has been variously explained by different authors. The fila- 

 ments of some Confer voids (Spirogyra, fig. 512, A), of Desm-idium, 

 &c., the families of cells of Palmellece (figs. 504, 513) and Nostoch- 

 inece, are surrounded by a coat of gelatinous consistence outside 

 the proper cell-membrane. This appears to be produced by the 

 softening and swelling up of the parent cells (of many generations) 

 of the cells which are surrounded by such envelopes. 



Cuticular Layers. Another layer is characteristic of many cell- 

 membranes which are destined to protect the subjacent tissues, 

 or their own contents, from, the action of the atmosphere, namely 

 those of epidermal cells and of pollen-grains and spores. These 

 exhibit a superficial pellicle, of varied character as to thickness, 

 texture, and marking, which pellicle appears subsequently to the 

 first formation of the cell. This, like the gelatinous coat just 

 described, is a structure altogether of secondary character, but is 

 distinguished from the ordinary secondary layers of thickening by 

 its position on the outside of the cell-wall. 



It is still a moot question whether these pellicles are secreted by the 

 primary membrane on the outside, or are formed by transformation of the 

 outer laminre of the primary membrane itself, whose place is then taken 

 by some of the outer secondary layers. This subject will be more dwelt 

 upon under the head of the cuticle. 



Thickening Structure. The secondary formations on the inside 

 of the cell-membrane may (1) correspond in character to the 

 primary wall, in which case the cell-wall is simply thickened by 

 new lamellae ; or (2) the new layers applying themselves over the 

 surface of the wall, leave certain parts bare, which appear as dots or 

 pits of various forms when viewed from the inside (figs. 531, A, B) ; 

 or (3) they are applied only over parts which form peculiar 

 patterns upon the primary wall, and appear, when of sufficient 

 thickness, like^&m adhering to it, spiral, annular, or connected 

 into a kind of network. 



Those secondary layers which resemble the primary wall, although 

 evenly deposited, present in certain cases an appearance as though 

 their molecules were arranged in a spiral order, since fine spiral streaks 

 may sometimes be detected, after treating them with acids and by other 

 means/ and many of them are apt to tear in a spiral direction. The 

 excessively delicate spiral marking here referred -to (seen in liber-cells of 

 Vinca, fig. 53.3, and most Apocynacese and AsclepiadaceaB, in wood- 

 cells of Pimis, in the cell-membrane of Hydrodictyon, &c.) must not be 

 confounded with a deceptive appearance, resembling a much coarser spiral 

 striation, produced by treating the membranes of Confeme, the paren- 



