CELL. 



[ 147 ] 



CELL. 



appears to be a land of goniclial reproduction, 

 in wliicli portions of llie living- contents 

 are enabled to save themselves from the 

 general decomposition. (See Pseudogo- 

 NiDiA.) Some forms -tt-hich we incline to 

 refer here, have been described as distinct 

 genera and species of parasitic Algre ; on 

 this subject see Parasites, Chytkidiuji, 

 Pythium, and Amy'lobactee. 



Memhraue. — Li all young organs of suc- 

 culent structures, and all tlie delicate tissues 

 of the higher plants, and in the majority of 

 the Cellular plants in almost the entire or- 

 ganization, the cellular membranes consist 

 of a thin structureless pelhcle, possessing a 

 considerable degree of toughness and a 

 certain amount of elasticity. (C. J. Agardh 

 has indeed asserted that cell-membrane is 

 composed of spiral librous structure ; but 

 this doubtless is an error as regards the pri- 

 mary membrane.) It is readily permeable 

 by vrater, while no orifices of any kind can 

 be detected in it ; but young, and indeed 

 soft cell-membranes generally, imljibe more 

 or less water, and swell to some extent, 

 often becoming more or less gelatinous. It 

 is stated by Schleiden that the membranes 

 of nascent cells are soluble in water, but 

 genei'al experience does not confirm this 

 statement ; the only approach to a corro- 

 boration of it that we have met with, is in 

 the lower Algte : the zoospores are often 

 extruded in the interior of an extremely 

 delicate sac formed of cellulose, which al- 

 most immediately vanishes and sets the 

 zoospores free. The external membranes 

 of many of the filamentous and unicellular 

 Algae become gelatinous, and giadually dis- 

 solve away as the inner membranes are 

 successively deposited, forming a gelatinous 

 coat, as in Prvtococcus, Nostoe, Desmidia- 

 ceae, Diatomaceae, Zyf/neona, Oscillatorieas, 

 &c. ; the same also takes place in the de- 

 velopment of spores and pollen-grains, which 

 are set free by the parent-cell membranes 

 becoming dissolved. This, however, is 

 scai'cely direct solution in water, and comes 

 rather under the head of decomposition. 



Young and delicate cell-membranes are 

 perfectly transparent and colom-less, as is 

 seen in the Yeast-plant, in the mycelium of 

 Mildews, in the cellular tissue of tuberous 

 structures like the Potato, and in piths (after 

 the mucilaginous cell-contents have been 

 removed). As they gi-ow older, they often 

 become coloured, sometimes very deeply, 

 which is suppu.-ed, howcAer, to depend on 

 the infiltration of foreign matters. In the 



state of simple cell- membranes, where no 

 infiltration of foreign matter has occurred, 

 the application of sulphuric acid of moderate 

 strength, with solution of iodine in solution 

 of iodide of potassium, brings out a bright 

 blue ciilour ; and this is regarded as a test 

 for cellulose, the universal basis of vegetable 

 cell-membrane. 



When the cell has attained a certain age, 

 new deposits of membranous substance take 

 place inside; and the walls thus acquire 

 more or less thickness, together with a 

 very varied appearance, according to the 

 character of the deposits. The new layers 

 are known as Secondary Layers ; and the 

 term Cell-wall is perliaps the most conve- 

 nient collective term which can be applied 

 to the various structures produced by the 

 deposition of new layers of cellulose upon 

 the inside of the primaiy cell-membrane. 

 Although these new deposits are thin layers 

 of cellulose hke the primary membrane, they 

 are rarely so totally devoid of detail struc- 

 ture, and in the majority of cases exhibit 

 orifices and irregularities of the most striking 

 character. Moreover, in one class of cases, 

 they are not deposited as a continuous coat, 

 but as a fibrous structure applied upon the 

 primary membrane, as in spiral-fibrous cells; 

 and in wood-cells they are formed one within 

 another to such a thickness that the cell- 

 wall loses its original membranous character, 

 and becomes a solid case, with the internal 

 cavity reduced to a comparatively small 

 chamber in the centre. 



The simplest condition of a thickened 

 cell-wall, is that met with in the unicellular 

 and filamentous Algfe, where the primary 

 membrane becomes coated in the interior by 

 successive continuous layers of cellulose 

 exactly resembling itself, and which often 

 indeed can only be known to exist by com- 

 paring the thickness of old and young cells, 

 since no lamellation can be detected ; gene- 

 rally speaking, however, the action of mode- 

 rately diluted sulphmic acid swells up such 

 membranes, and renders the lamellas more 

 or less distinct (Pi'. 47. fig. 24). The thicken- 

 ing layers of the unicellular and filamentous 

 Algte 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 mauj' cases set free by 

 the solution of the latter. These cell-walls 

 sometimes exhibit peculiar fibrous appear- 

 ances. See Spiral Structvees. 



These layers may be coloured blue by 



l2 



