154 THE CELL 



may be utilised by the formative activity of the cell, in the same 

 way as has been suggested above, that other substances are used 

 for the formation of the cellulose membrane. 



In what way the protoplasm executes its above-mentioned 

 function of adoption is quite beyond our comprehension at this 

 present time, when the majority of the bio-chemical processes 

 escape our observation. This function of the protoplasm, however, 

 may consist in this, that certain particles of its substance may 

 unite, through molecular addition, with particles of other sub- 

 stances present in the nutrient solutions, and thus become trans- 

 formed into an organic product. Thus soluble silicious compounds 

 may unite with molecules of organic substance to form a silicious 

 skeleton ; thus particles of cellulose may be formed through the 

 influence of particles of protoplasmic substance from soluble 

 carbo-hydrates, forming with them a compound (probably per- 

 manent, but possibly only temporary), and becoming organised to 

 form a cell-membrane. This conception is quite in accordance 

 with the fact that in many objects freshly-formed layers of 

 cellulose are found to pass imperceptibly into the neighbouring 

 protoplasm. 



2. The Morphology of Metabolism. The formative activity of 

 the Cell. The substances which are formed during the meta- 

 bolism of the cell may be included under the head of morpho- 

 logy > i n so far as they can be optically distinguished from the 

 protoplasm. They may be differentiated out in a formed or 

 unformed condition, either in the interior of the protoplasm, or 

 upon its surface ; according to their position they are distin- 

 guished as internal or external plasmic products. However, as is 

 so often the case in biological classifications, a sharp line of dis- 

 tinction cannot be drawn between the two groups. 



a. Internal Plasmic Products. Substances dissolved in water 

 may separate out as larger or smaller drops in the protoplasm, 

 and thus cause cavities or vacuoles. These play a most important 

 part, especially in the morphology of plants. As has already been 

 described in detail on p. 31, a plant cell (Fig. 62) is able by 

 secreting sap to increase its size in a short time more than a 

 hundred-fold. It is by means of the simultaneous action of a 

 large number of such cells that in spring-time certain organs of 

 plants are able to grow to such a considerable size. The solid 

 substance contained by a plant very rich in water may be as little 

 as 5 per cent., or even only 2 per cent. 



