ELEMENTARY STRUCTURE. 



59 



becomes gradually constricted on all sides {fig. Ul, a,b,c,d), 



thus folding inwards, and ultimately dividing by a kind of 



Fig. 141. 



ah c d 



Fig. 141. a. Cell of Conferva glomerata, with the cell-contents constricted 

 by the half-completed septum, b. A half-completed septum in whicli a 

 considerable deposition of cellulose has already taken place, c. A septum 

 in course of development, after the action of an acid, which lias caused 

 contraction both of the primordial utricle (a) and the cell-contents (6). 

 d. Complete septum split into two lamellae by the action of an acid. 

 After Mohl and Henfrey. 



hour-glass contraction the original primordial utricle and the 

 contents of the cell Avithin it into two distinct portions, each 

 portion of the primordial utricle then secretes a layer of cellulose 

 over its whole surface, and where these portions are in contact 

 "VN-itli the original wall of the primary cell they only form new 

 layers of thickening to it, but when separated from the wall, as 

 is the case where the infolding takes place, they form distinct 

 cellulose membranes, which will be, however, continuous with 

 those layers of thickening. The original cell thus becomes 

 divided into two; each of these then has the power of growing 

 until it reaches the original size of its parent, and then either, or 

 both, may again divide in a similar manner until the plant, or 

 organ, of which they form a part, is completed. 



In this mode of cell- formation, it is by no means evident what 

 function the nucleus performs. That this is unimportant is 

 clear, because cell-division as above described may take place, 

 as it does in some of the lower orders of plants, -without the pre- 

 sence of that body. In the higher orders of plants, however, 

 the original nucleus of the cell appears to undergo subdivision 

 into two halves, as is the case with the other contents, so that a 

 nucleus is thus formed for each new cell into Avhich the parent 

 cell has been divided. In other cases, however, separate nuclei 

 are formed for the secondary cells, instead of the original 

 nucleus dividing into two. 



In some of the lower kinds of plants, a modification of this 

 process of cell-division lakes place ; it consists in the formation 

 of the secondary cells, as little bud-Uke prominences on the sur- 

 face of the primary cells, either at their extremities, as in the 

 Yeast plant (fig. 142), &c., by which the plant is increased in 

 length ; or on the side of the primary cell when branches are 

 produced, as in some Confervie (fig. 143), in the fibrilliform cells 



