CELL. 



[ 123 ] 



CELL. 



brane becomes coated in the interior by suc- 

 cessive continuous layers of cellulose exactly 

 resembling itself, and which often indeed 

 can only be known to exist b}' comparing the 

 thickness of old and young cells, since no 

 lamellation can be detected; generally speak- 

 ing, however, the action of moderately di- 

 luted sulphuric acid swells up such mem- 

 branes, and renders the lamellae more or less 

 distinct (PL 38. fig. 24). The thickenmg 

 layers of the unicellular and filamentous 

 Algse are scarcely to be compared with those 

 of the cells of higher plants, since they are 

 rather to be regarded as the primary mem- 

 branes of new cells produced in the interior 

 of the older cells, in many cases set free bj? 

 the solution of the latter. These cell-v.alls 

 sometimes exhibit peculiar fibrous appear- 

 ances. See Spiral Structures. 



These layers may be coloured blue by sul- 

 phuric acid and iodine when young, but 

 where they undergo solution into a kind of 

 jelly, as in filamentous Algre, this cellulose 

 reaction seems to fail ; at all events, it is so 

 uncertain in its behaviour, that although it 

 gives a positive result in successful cases, a 

 negative result is altogether inconclusive. 



In the cells of the generality of ])lants of 

 higher organization, the secondary cell-mem- 

 branes exhibit a striking difference from the 

 primary, inasmuch as we find them con- 

 stantly perforated by holes, slits, or orifices 

 of some shape, so as to leave the primary 

 membrane bare, whence results a spotted 

 or streaked appearance of the cell-wall, as 

 may be seen even in cells with the walls still 

 very thin, such as fully-formed pith-cells of 

 the Elder. 



The earlier anatomists regarded these 

 spots or dots as orifices through the cell- 

 wall, but they are in reality only pits open- 

 ing into the ca^ity of the cell, and closed at 

 the bottom by the original external mem- 

 brane of the cell. When the cell-wall be- 

 comes much thickened, as in cells of horny 

 albumen or wood-cells, the layers success- 

 ively deposited over the inside, mostly cor- 

 respond pretty exactly with the earliest 

 layers, and leave the spots always free, so 

 that these become gradually converted into 

 tubular canals running through the thick 

 cell-walls (PI. 38. figs. 21-23 and 27). In the 

 majority of cases, if not in all, the spots or 

 pits in the cell-wall are op])Osite to similar 

 spots in the walls of the adjacent cells, so 

 that the cavities of the two contiguous cells 

 are only separated from each other by the 

 primary membrane of each;, as at first, allow- 



ing free permeation of fluid from one to the 

 other. In old cells these primary membranes 

 become destroyed, and thus the cavities com- 

 municate freely through these canals run- 

 ning out through their hard thickened walls. 

 The various complications of these pits are 

 spoken of under the head of Pitted cells. 



The secondary layers are further distin- 

 guished from the primary membrane by the 

 prevalence of a tendency to assume the cha- 

 racter of spiral bauds or fibres winding upon 

 the original cell-wall. This may be detected 

 even in many cells which remain quite mem- 

 branous, as in some Confervce and many 

 hairs, also in pitted lignified cells, where the 

 thickening layer forms a general coat upon 

 the inside of the cell; for the liber-cells of 

 many plants exhibit a dehcate spiral striation 

 of their walls, while some liber-cells display 

 it with especial distinctness. Some of these 

 cells give way in a spiral direction when torn 

 b}' pulling lengthways. In parenchymatous 

 cells this spiral structure is often very fully 

 developed in all its varieties ; but it is espe- 

 cially characteristic of the vessels and ducts, 

 while in certain woods, as in Taxus, we 

 have a combination of the porous with the 

 spiral secondar}- deposits, the earlier thick- 

 ening layers leaving spots uncovered while 

 the later ones are deposited along a spiral 

 line coiling up the cell-wall from bottom to 

 top, and thus the cell appears to have a spi- 

 ral fibre lying upon its walls. These struc- 

 tures are spoken of at length under the head 

 of Spiral deposits and Pitted cells. 



Cellulose is distinguished in vegetable 

 structm-es by the blue colour it assumes with 

 aqueous solution of iodine after treatment 

 with sulphuric acid. The nitrogenous pro- 

 toplasm is always coloured yellow-brown by 

 this means. It is sometimes difficult to 

 bring out the blue reaction in old cells; 

 various methods are had recourse to for this 

 purpose. In cork)- or other epidermal tis- 

 sues, the blue colour of cellulose may be 

 brought out by soaking the cells for twenty- 

 four hours or more in strong solution of 

 potash, washing it well, soaking in tincture 

 of iodine, drying, and then wetting with 

 water. Old wood-cells undergo the same 

 change by boiling in nitric acid, instead of 

 treating with caustic potash, and then adchng 

 the iodine, &c. as above. All the solid 

 structures of cell-membranes yield to one or 

 other of these means, and exhibit the blue 

 colour with iodine, which if not indicative of 

 a composition of cellulose, points to a sub- 

 stance intermediate between this and starch. 



