CELL. 



[ 144 ] 



CELL. 



as in Cotton, where a single filamentous 

 cell may measiu'e 1 to 2". (See Tissues, 

 Vegetable.) 



Cells may be examined either in situ, as 

 parts of tissues, or free, separated naturally 

 or artificially. For the lirst it is simply 

 requisite to make fine slices with a razor, in 

 various directions through the structure ; if 

 soft or thin, it should he placed between the 

 two halves of a split vial-cork and sliced with 

 the cork, the cork being afterwards removed 

 from the slide with a needle. Slices of many 

 kinds of cellular tissue are made more clear 

 by the addition of a little diluted sulphuric 

 acid, which, however, often swells up some 

 of the layers. 



For examining isolated or nearly separate 

 cells, we may take the lower Algae or Fungi, 

 or germinating spores of the higher plants, 

 or we may separate the cells of the tissues of 

 higher plants. The parenchymatous tissues 

 may usually be separated into their elemen- 

 tary cells by maceration in water : the de- 

 composing ends of floAver-stalks which have 

 been in water several days will generally 

 afford tissue in such a state that it may be 

 broken up with a needle ; in the pulp of ripe 

 fruits, such as currants, strawberries, &c., 

 mere pressure separates the cells. Boiling 

 will do with some of the denser kinds ; 

 while for the woody tissues it is requisite to 

 heat fragments with a particle of chlorate 

 of potash and a drop of nitric acid (or let 

 them macerate for 12 to 24 hours), and wash 

 them well with water: liber-cells, woody 

 cells, &c. may be isolated by this means ; 

 or still better by treating them with chromo- 

 sulphuric acid. 



Formation of Cells. — This subject has 

 undergone a great amount of investigation 

 during the last few years ; and the views 

 which have been propounded at various 

 times have conflicted strongly in many 

 points. It would be exceeding our limits, 

 however, to enter upon a critical examina- 

 tion of the theories of cell-development; 

 and we shall therefore conlhie ourselves to 

 a brief account of those phenomena and 

 laws of the reproduction of cells upon 

 which the diversity of opinion only afl'ects 

 subordinate particulars. 



All vegetable cells (using this term in the 

 sense of the cellulose sac loit.h contents, as de- 

 fined above) in which the capacity for repro- 

 duction exists, contain an internal structure 

 or protoplasm, varying in its condition and 

 appearance at different epochs and in dif- 

 ferent plants or parts of plants. This struc- 



ture often forms a semifluid layer lining the 

 cellulose wall of the cell. All the other 

 cell-contents are enclosed or imbedded in 

 this protoplasm ; and with it they collectively 

 constitute what is called by some authors 

 the endochrome of the cell. 



In a very large number of cases, we find 

 in the protoplasm at this time, a peculiar 

 body called the nucleus, to which some 

 writers attribute great importance in the 

 dovelopment of cells. Its nature is not 

 well defined ; but in the best-observed cases, 

 it consists of a small globular or lenticular 

 mass, apparently composed of the protoplasm 

 in a condensed and granular (solid) con- 

 dition. The nucleus is usually present in the 

 cells of the higher plants, but is often ab- 

 sent in those of the ThaUophytes. It mostly 

 exhibits one or more bright granules or 

 points in its substance, which are called 

 nvclcoli. Many authors consider this body 

 of the first importance in cell-development ; 

 but as we are by no means satisfied as to 

 the character of its agency, its peculiarities 

 and its relation to the cell are spoken of 

 separately under the head of Nucleus 

 (PL47. figs. 8,9 w). 



The ordinary development of new cells 

 depends upon the division of the protoplasm 

 of existing cells into two or more portions, 

 which, becoming independent centres of 

 life, produce new cellulose membranes, and 

 become new cells. The phenomena in which 

 this law is manifested are far more varied 

 tlian would be iinagined from this simple 

 statement. The numerous subordinate mo- 

 difications, however, maybe arranged under 

 three principal lieads : — 1. Cell-division, 

 sometimes called ;«f/7'sw;«^/c cell-formation; 

 2. Cell-division with liberation of the new 

 cells ; 3. Free cell-formation. 



1. Cell-division is the process which occurs 

 in all reproduction of cells connected with 

 vegetative growth or increase of the mass of 

 existing structures. This is the manner in 

 which the cells are multiplied in the growth 

 of the thallus of the interior plants, and in 

 the growth of the stems, leaves, roots, and 

 other organs of the higher plants. It occurs 

 also in the formation of the basidiospores or 

 sti/loKpores of Fungi, the spcrniiifi'a of these 

 and Lichens, of (/ontdia in the Lichens, and 

 conidia in the Fungi. The essential fact 

 observed in all the cases is, the division of 

 the protoplasm of the parent-cell into two 

 or more distinct portions, each of which 

 secretes a layer of cellulose over its whole 

 surfece ; and thus, when the two are in ap- 



