13 



CHAPTER II. 



THE STUDY OF A LIVING ASSIMH -"^t — CELL. 

 A. Th^ lally Differentiated Assimilating Cell. 



Set'ORE passing on to the consideration of the various modifica- 

 tions which are met with in plant cells, it is advisable to examine 

 a typical living cell in which some of the more well-defined vital 

 jjrocesses may be easily demonstrated. Such cells are to be 

 found in the green assimilating tissues of plants, such as the 

 mesophyll of leaves, and the outer part of the cortex of herb- 

 aceous stems. 



ValUsneria spiralis, a water-plant, affords very good material to 

 work with in this respect, as the cells of the leaf of this plant are 

 typical assimilating cells, the term assimilation being understood 

 in its true botanical sense, as, for example, in the taking in of 

 carbon dioxide and water, and the elaboration of these into 

 carbon-compounds in the chloropbyll bodies, oxygen being evolved 

 during the process. 



If a leaf of ValUsneria be taken, and a small portion of it 

 mounted in water and examined under the half-inch power of 

 the microscope, the following details may be made out by focuss- 

 ing into various planes : — 



i. The outermost layer of the leaf, composed of elongated cells rect- 

 angular in shape, and forming the epidermis. 



ii. Internally as regards these, somewhat elongated cells rounded off 

 at the angles : it is with these cells for the most part that the leaf carries 

 on the process of assimilation. 



iii. Smaller cubical cells, which occur near the edges of the leaf. 



Using a higher power of the microscope (■^" objective) it is 

 possible to distinguish in any of these cells (i. or ii.) the follow- 

 ing features (see Fig. 3) : — 



(a) The eell-wall, a delicate membrane enclosing the other parts of 

 the cell or cell-contents. 



