80 Vcfjetahle Physiulof/y. 



of firm substance of the same nature as the outer cell-membrane, 

 and thus the two new bags which it forms Vjecome encased in 

 proper firm coats, which render the walls of the old part of 

 the cells thicker, and form a double partition at the part where 

 the division took place. 



We now see how the cell of the Protococcus divides into foui", 

 and how the multiplication of cells generally is effected. The 

 cell of Protococcus is filled with protoplasm (containing green 

 colouring matter). When the cell is about to divide, this pivto- 

 plasm breaks up into two or four portions, each of which coats 

 itself with a new membrane, and thus becomes a cell. As these 

 grow, they are, of course, at first confined by the membrane of 

 the original cell : this either stretches, dissolves away, or cracks 

 and peels off, in different plants. In Protococcus it dissolves ; in 

 a little plant of similar nature, called Schizochlamys, it cracks 

 and falls off. In ordinary vegetable substances it expands, being 

 stretched by the swelling of the new cells within, until it becomes 

 so thin as to be invisible. Thus, in the Spiroyyra before men- 

 tioned we may trace the successive encasings of the cells, the 

 oldest membrane, stretched by the growth of the new cells within, 

 becoming gradually so thin that it cannot be distinguished.* 



We have said that all growth of plants takes place by means 

 of the division of cells in this way. In ordinary cases the new 

 cells are consequently at first of the shape of a segment of the 

 old one ; but in certain structures this is somewhat modified, or 

 at all events the old cell, when dividing into two, may produce 

 two dissimilar cells. This takes place in the formation of free 

 tellular structures, such as the branches of cellular plants, the 

 formation of hairs, &c., and in the increase of number of a few 

 plants. Thus, in the branching of some Confervoids, and of the 

 filamentous structure growing from the ' spores' of Mosses, the 

 parent cell grows out at the side, and then this lateral process is 

 shut off as a new cell. In the growth of the yeast-plant the new 

 cells bud out from the sides of the old ones, and are at length shut 

 off and detached. (Fig. 9.) 



Moreovei', cells are in certain cases developed in a manner 

 slightly different from that above described, namely, in the forma- 

 tion of the cells which are to produce new separate plants ; but 

 it is onlv by a modification of the foregoing process. In that the 

 whole contents of the cell became parted into two or four 

 portions to constitute the contents of two or four new cells. 



* This encasing of the cells is beautifully illustrated by what occurs when we 

 allow dead filaments of Spiroijtjm to remain in water. As the firm coat decays 

 from without inwards, the lamina; are successively dissolved, and the filament 

 breaks into strings of eight, tlien of four cells; these fall into pairs, and finally 

 the youngest generation remains alone in the state of single cells. 



