CELL. 



[ 148 ] 



CELL. 



sulphuric acid and iodine; wlien very young, 

 even by iodine alone; but when old or where 

 they undergo spontaneous solution into a 

 kind of jelly, as in lilamentous Algse, this 

 celliUose reaction seems to fail — at all events 

 it is so uncertain in its behaviour, that, 

 although it gives a positive result in suc- 

 cessful cases, a negative residt is altogether 

 inconclusive. 



In the cells of the generality of plants of 

 higher organization, the secondary cell- 

 membranes exhibit a striking difference 

 from the primary, inasmuch as we find them 

 constantly perforated by holes, slits, or 

 orifices of some shape, so as to leave the 

 primary membrane bare ; whence resvdts 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 Polder. 



The earlier vegetable anatomists regarded 

 these spots or dots as orifices through the 

 cell- wall ; but they are in reality only j^its 

 opening into the cavity of the cell, and closed 

 externally by the original membrane of the 

 cell. "When the cell-wall becomes much 

 thickened, as in cells of horny albumen or 

 wood-cells, the layers successively deposited 

 over the inside mostly correspond pretty 

 exactly with the earliest layers, and leave 

 the spots always free, so that these become 

 gradually converted into tubular canals run- 

 ning through the thick cell-walls (PI. 47. 

 figs. 21-2o and 27). In the majority of 

 cases, but not in all, the spots or pits in the 

 cell-wall are opposite 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, allowing free 

 permeation of fluid from one to the other. 

 In old cells these primary membranes be- 

 come destroyed, and thus the cavities 

 communicate freely through these canals 

 running 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 

 character of spiral bands or fibres winding 

 upon the original cell-wall. This may be 

 detected even in many cells which remain 

 quite membranous, 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 ; the liber- 



cells of many plants exhibit a delicate 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 by pulling lengthwise. In par- 

 enchymatous cells this spiral structure is 

 often very fully developed in all its varieties ; 

 but it is especially characteristic of the 

 vessels and ducts ; while in certain woods, 

 as in Taxus, we have a combination of the 

 porous with the spiral secondary deposits, 

 the earlier thickening layers leaving spots 

 uncovered while the latter ones are deposited 

 along a spiral line coiling up the cell-wall 

 from bottom to top, and thus the cell 

 appears to have a spiral fibre lying upon its 

 walls. These structures are spoken of at 

 length under the heads of Splr^vl deposits 

 and Pitted cells. 



Cellulose is distinguished, when in the 

 form of membrane or fibrous structure, by 

 the blue colour it usually assumes when 

 treated with iodine (starch differs in its 

 granular form and its solubility in acids and 

 potash, and its swelling up in hot water). 

 The nitrogenous protoplasm is always co- 

 loured yellow-brown by iodine. The blue 

 colour appears in many membranous paren- 

 chymatous tissues when the cells are soaked 

 in tincture of iodine, dried, and then wetted 

 with water. In other cases it is necessary 

 to apply dilute sulphuric acid and solution 

 of iodine simultaneously. It is sometimes 

 difficult to bring out the blue reaction in 

 old cells ; various methods are had recourse 

 to for this purpose. In corky or other epi- 

 dermal tissues, the blue colour of cellulose 

 may be brought out by soaking the cells for 

 twenty-four liom-s or more in strong solution 

 of potash, washing it well, soaking in tinc- 

 ture 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 

 adding 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 substance intermediate between 

 this and starch, produced out of the cellu- 

 lose by the chemical action. The most 

 characteristic property of cellulose, how- 

 ever, is its solubilit}' in ammuniin-et of 

 copper (Cellulose.) The cells of Fungi 

 and many Lichens and Algse do not exhibit 

 the ordinary reactions of cellulose, becoming 

 brown instead of blue with iodine and sul- 



