STRUCTURE OF THE PLANT-CETL 3 



of the white of an unboiled egg, and usualh' contains large 

 numbers of small granules which are partly of the nature of 

 food-bodies and partly waste-products. The fact that not all, 

 though some, of the properties of living protoplasm are exhibited 

 by cells which are ground to pulp suggests that the particular 

 characteristics of the cytoplasm are not entirely an outcome of 

 its chemical constitution, but are to some extent a consequence 

 of its ultimate structure. The nucleus is the most important 

 part of the protoplast, a fact which will become more apparent 

 in later chapters (Part II), when the reproductive processes of 

 plants are studied. It appears to be essential for many 

 of the vital activities of the cell. A demonstration of this is 

 afforded by the fact that, if the unicellular animal Stenlor is 

 broken into pieces, the fragments which contain portions of the 

 nucleus develop into new individuals, whilst the others, after 

 a short time, perish. Moreover, the nucleus is generall}' found 

 in that part of the cell which is in process of active growth, 

 e.g. at the tip of a growing root-hair. 



The structure of the Onion-cell can be more clearly distin- 

 guished if the strip of epidermis be stained with a drop of iodine 

 (see Appendix II), and a single cell examined under the high- 

 power objective. The protoplasm will have assumed a yellowish 

 tint, whilst the nucleus appears pale brown. This colour 

 reaction of the protoplast is one characteristic of proteins gener- 

 ally. The nucleus (Tig. i, B, X) is bounded by a thin nuclear 

 membrane, whilst within it one or two small round bodies, the 

 nucleoli [n), are now plainly visible, since they are stained more 

 deeply than the rest. The cytoplasm does not completelj' lill 

 the cell, but there is a large space or vacuole {Va.) occupj-ing the 

 greater part of the central region ; this vacuole, apparently 

 empty, is in reahtj" filled with a watery fluid, the cell-sap. Close 

 observation shows that the cytoplasm is not evenly granular 

 throughout, but that there is a very thin clear layer immediately 

 within the cell-wall. This layer is a result of physical forces 

 operating at the surface of the cytoplasm, and a similar clear 

 layer can be detected at the surface abutting on the vacuole. 

 These layers are spoken of as the plasmatic membranes. 



If another strip of Onion epidermis be mounted in concen- 

 trated sulphuric acid, the cell-walls wiU swell and disappear. 



