THE ASSIMILATING CELL. 15 



(ft) A layer of ppotoplasm, lining the inner surface of the cell- wall.* 



(c) The nucleus, lying somewhere in this layer. 



id) Numerous oval green ehloroplaStS, also lying embedded in the 

 protoplasm. 



(e) The central vacuole, filled with cell-sap, enclosed by the 

 protoplasmic sac; in the smaller cells several vacuoles may be present. 



These several parts should now be examined in detail ; and 

 for this purpose it is as well to use a small " stop " on the sub- 

 stage diaphragm of the microscope (or on the iris-diaphragm 

 often fitted to the condenser) in order to cut off the peripheral 

 illuminating rays, and thus obtain a very much sharper definition 

 of each object examined. 



With these precautions it is possible to make out that the 

 cell-wall is a delicate membrane of a transparent homogeneous 

 material : in this case it is not always possible to make out that 

 the boundary-wall of adjacent cells is in reality double, unless 

 very careful focussing is made, but that this is so will be readily 

 seen in many other tissues which will be examined further on. 



The inner edge of the layer of protoplasm is now more 

 distinct, and the protoplasm itself is seen to be composed of a 

 clear substance in which are suspended the chloroplasts, and some 

 small granules, these latter being either of a protoplasmic 

 nature! or food-granules (starch, etc.). 



Lying in the protoplasm, and, as a rule, close to the cell-wall, 

 is the nucleus, an ellipsoidal body with a centrally situated 

 round spot, the nucleolus ; the main substance of the nucleus in 

 the living cell appears to be nearly homogeneous, but certain 

 reagents, such as acetic acid, show up a distinct reticulum, and 

 some stains, notably safranin and haematoxylin bring out other 

 features, which will be examined in Chap. viii. 



By the time these observations have been completed, there 

 will probably have occurred a phenomenon which first appears in 

 the more internal rectangular cells. If closely watched the 

 protoplasm of some of these cells will be seen to be moving 

 slowly round the cell, carrying with it granules, nucleus and 

 chloroplasts. This movement is known as " rotation " or 

 " streaming," and up to a certain point its rate increases with 

 the temperature; it is the endoplasm which really moves, the 



* This layer is the "primordial utricle " (}jrimordialschla%ich) of von 

 Mohl. 



t So-called "microsomata." 



