390 



DEVELOPMENT OF VEGETABLE CELLS. 



folia). Such cells usually contain evident Nuclei, which are turned 

 brownish-yellow by iodine, whilst their membrane is only turned 

 pale-yellow ; and in this way the Nucleus may be brought into 

 view, when, as often happens, it is not previously distinguishable. 

 If a drop of the iodized solution of chloride of zinc be subsequently 

 added, the cell-membrane becomes of a beautiful blue colour, whilst 

 the nucleus and the granular protoplasm that surrounds it retain 

 their brownish-yellow tint. The use of dilute nitric or sulphuric 

 acid, of alcohol, of syrup, or of several other reagents, serves to 

 bring into view the Primordial Utricle (§ 182) ; its contents being 

 made to coagulate and shrink, so that it detaches itself from the 

 Cellulose wall with which it is ordinarily in contact, and shrivels-up 

 within its cavity, as shown in Fig. 197. It would be a mistake, 

 however, to regard this as a distinct membrane ; for it is nothing 

 else than the peripheral layer of Protoplasm, naturally somewhat 

 more dense than that which it includes (like the Ectosarc of 

 Rhizopods (§ 325), but deriving its special consistence from the 

 operations of Reagents. 



286. Although the usual mode of increase in the higher Plants 

 is by the multiplication of Cells by binary subdivision, after the 

 plan which we have seen to prevail among the lower, yet there is 

 evidence that, in the formation of new parts, cells may spring up 

 as it were cle novo, by a process of gradual differentiation in a Pro- 



Successive stages of Cell-formation in the development of the 

 Leaves of Anacharis alsinasirum : — a, growing point of the 

 branch, consisting of a protoplasmic mass with vacuoles, the pro- 

 jections at its base being the rudiments of Leaves ; b, portion 

 of one of these incipient Leaves in a more advanced condition ; 

 c, the same in a still later stage of development. 



