226 THE MICROSCOPE AND ITS REVELATIONS. 



closis ' hereafter to be described ( 258) is carried-on within the Plant-cell, 

 which is the most interesting to the Microscopist of all its manifestations 

 of vital activity. The nucleus is a small body, usually of lenticular or 

 sub-globose form (Fig. 139, A, a), and of albuminous composition, that 

 lies imbeded in protoplasmic substance, either on the cell- wall or in the 

 central cavity. It is not, however, constantly present even in the higher 

 forms of cell-structure ; for in those cells whose active life has been com- 

 pleted, the nucleus is usually absent, having probably been resolved again 

 into the protoplasm from which it was originally formed. And in the 

 cells of many of the lower Cryptogams, it cannot be distinguished at any 

 stage of their existence. Within the nucleus are often seen one or more 

 small distinct particles termed nucleoli (Fig. 139, A, Z), which can be 

 best distinguished by the strong coloration they receive from a 24 hours' 

 immersion in carmine, and subsequent washing in water slightly acidu- 

 lated with acetic acid. Though the precise function of the nucleus is 

 still unknown, there can be no reasonable doubt of its peculiar relation 

 to the vital activity of the cell: for in the nucleated cells which exhibit 

 'cyclosis,' it may be observed that if the nucleus remains attached to the 

 cell-wall, it constitutes a centre from which the protoplasmic streams 

 diverge, and to which they return; whilst if it retains its freedom to wan- 

 der about, the course of the streams alters in conformity with its position. 

 But it is in the multiplication of cells by binary subdivision which 

 will be presently described ( 226), that the speciality of the nucleus as 

 the centre of the vital activity of the cell is most strongly manifested. 

 The chlorophyll corpuscles, which are limited to the cells of the parts of 

 plants acted-on by light, are specialized particles of protoplasm through 

 which a green coloring matter is diffused : and it is by them that the 

 work of decomposing CO 2 , and of ' fixing' its carbon, by union with the 

 oxygen and hydrogen of water, into starch (which seems to be the basis 

 of all other vegetable compounds), is effected. The characteristic green 

 of chlorophyll often gives place to other colors, which seem to be pro- 

 duced from it by chemical action. Starch-grains are always formed in 

 the first mtance in the interior of the chlorophyll-corpuscles, and gradu- 

 ally increase in size until they take the places of the corpuscles that pro- 

 duced them. So long as they continue to grow, they are always imbeded in 

 the protoplasm of the cell ; and it is only when fully formed, that they 

 lie free within its cavity (Fig. 246). 



225. But although these component parts may be made-out without 

 any difficulty in a large proportion of Vegetable Cells, yet they cannot be 

 distinguished in some of those humble organisms which are nearest to 

 the border-line between the two Kingdoms. For in them we find the 

 ' cell- wall ' very imperfectly differentiated from the ( cell-contents;' the 

 former not having by any means the firmness of a perfect membrane, and 

 \the latter not possessing the liquidity which elsewhere characterizes 

 them. And in some instances the cell appears to be represented only by 

 a mass of endochrome, so viscid as to retain its external form without 

 any limitary membrane, though the superficial layer seems to have a 

 firmer consistence than the interior substance; and this may or may not 

 be surrounded by a gelatinous-looking envelope, which is equally far 

 from possessing a membranous firmness, and yet is the only representative 

 of the cellulose-wall. This viscid endochrome consists, as elsewhere, of 

 a colorless protoplasm, through which minute coloring particles are 

 diffused, sometimes uniformly, sometimes in local aggregations, leaving 

 parts of the protoplasm uncolored. The superficial layer, in particular, 



