534 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



the absence of granules ; and it is shown by the effects of reagents to 

 have the all> miiin rms composition of protoplasm. It may tlius l>e 

 regarded as the slightly condensed external film of the protoplasmic 

 layer with which the inner surface of the cell-wall is in contact ; and 

 it " essentially corresponds to the 'ectosarc' of Antnlxi or any other 

 rhizopod. The ' ectoplasm ' and ' cellulose wall ' can be readily dis- 

 tinguished from each other by chemical tests, and also by the action 

 of carmine, which stains the protoplasmic substance (when dead) 

 without affecting the cellulose wall. The further contents of the cell 

 consist of a watery fluid called cell sap, which holds in solution sup ir. 

 vegetable acids, saline matters, &c. ; the peculiar body termed the 

 nucleus ; and chlorophyll corpuscles (enclosing starch granules), 

 oil particles, &c. In the young state of the cell the whole cavity 

 is occupied by the protoplasmic substance, which is, however, viscid 

 and granular near the cell-wall, but more watery towards the interior. 

 With the enlargement of the cell and the imbibition of water, clear 

 spaces termed vacuoles, filled with watery cell-sap, are seen in the 

 protoplasmic substance; and these progressively increase in size and 

 number, until they come to occupy a considerable portion of the 

 cavity, the protoplasm stretching across it as an irregular network 

 of bands. Each of the vacuoles is enclosed in a very delicate con- 

 tractile membrane, the tonoplast. When, as usually happens, the 

 nucleus lies imbedded in the outer protoplasmic layer, these bands 

 are gradually withdrawn into it, so that the separate vacuoles unite 

 into one large general vacuole which is filled with watery cell-sap. 

 But where the nucleus is situated nearer to the centre of the cell, 

 part of the protoplasm collects around it, and bands or threads of 

 protoplasm stretch thence to various parts of the parietal layer. It 

 is by the contractility of the protoplasmic layer that the curious 

 ' cyclosis' hereafter to be described 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 subglobose form (fig. 413, A. fi). and of 

 albuminous composition, that lies imbedded in protoplasmic sub- 

 stance, either close to the cell-wall or nearer the centre of the 

 cavity. Cells containing a number of nuclei, or ' multinucleaied cells J 

 are not uncommon. They occur, for example, in many alga\ in the 

 'suspensor' and 'embryo-sac' of the ovule of phanerogams, and in 

 the ' laticiferous ' tubes. Within the nucleus are often seen one or 

 more small distinct particles termed nucleoli (fig. 413, A, b), which 

 can be best distinguished by the strong coloration they receive from 

 a twenty-four hours' immersion in carmine, and subsequent washing 

 in water slightly acidulated with acetic acid. Though in some points 

 the precise function of the nucleus is still unknown, there can be no 

 doubt of its essential relation to the vital activity of the cell, at least 

 in all the higher plants, although in the cells of some of the lower 

 cryptogams it has not at present been distinguished with certainty 

 at any stage of their existence. 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 



