THE LIVING CELL 31 



in size and complexity is another feature of the cell, the materials 

 for it being prepared by the metabolic activities mentioned. Finally 

 the living cell is sensitive or irritable to certain outside influences 

 which act as stimuli. Irritability is most conveniently discussed in 

 relation to whole organs of plants, and is considered in Chapter IX. 



In order that the cell may display its full activity, certain conditions 

 must be provided. Suitable raze materials must be present before 

 constructive processes and growth can occur, while a source of energy 

 must be to hand. For most purposes this energy is derived from the 

 oxidation of sugars previously built up in photosynthesis, and so the 

 presence of oxygen is for most cells a necessary condition to normal 

 activity. In the particular constructive process known as photo- 

 synthesis, light energy is utilised, and thus light must be available 

 for this process to be carried out. The cell structures normally 

 contain a high proportion (over 80 per cent.) of water and adequate 

 supplies of this must be available. Its absence leads to death or to a 

 suspension of cell activities, as in dormant seeds where the cells are 

 in a state of desiccation. A suitable temperature must be provided, 

 as mentioned in Chapter I. In most plants vital activity is only 

 manifest between 0° C. and about 45 ° C, with greatest activity pre- 

 vailing between 25 C. and 35 C. ; exposure of cells to temperatures 

 above or below the former range is liable to result in death (see also 

 Chapter IX.). Vital activity is suspended by exposure of cells to 

 substances such as chloroform or ether, which act as narcotics. From 

 the effects of these the cells may subsequently recover, while other 

 substances such as alcohol, or compounds of copper, act as poisons 

 and kill the cell. 



The operation of some of these factors can conveniently be studied 

 in observations on the protoplasmic streaming, which, as already 

 mentioned, is shown by some cells. The rate of streaming serves as 

 an index of the general activity of the cell. Low temperatures slow 

 down the streaming, higher temperatures accelerate it until a certain 

 temperature (the optimum) is reached, above which the cell is injured 

 and the movements soon cease. Deprival of oxygen or exposure to 

 narcotics or poisons causes the movements to stop. 



Protoplasm. 



AH these attributes of the cell — metabolism, growth, irritability — 

 have their genesis in the protoplasm. We must therefore enquire 

 why such remarkable properties should reside in the protoplasm. 



