THE VEGETABLE CELL 295 



processes carried out by the latter vary very much in different 

 cells, the cell-sap has no constant composition, but varies very 

 much in different cells, according to the work they do. 



The substances which the protoplasm constructs are again 

 destined to different purposes. Some are to be used up in various 

 ways, either in the cell in which they are formed, or in other 

 cells to which they are to be transported. These are generally 

 known as reserve materials. They may remain in the cells for 

 a longer or shorter period. Others are to be regarded as mate - 

 rial which is of no farther use to the plant at all, and are there- 

 fore to be got rid of. It is important to remember that the 

 Ijlant, unlike the animal organism, has little or no power to 

 excrete anything from its body. Waste products, therefore, 

 instead of being thrown off at once by definite channels, are 

 usually packed away in particular cells of the plant — for instance, 

 in the bark of trees. 



Other products are neither reserve nor waste materials, but 

 are formed in the cells to perform definite work there ; such are 

 the bodies known as enzymes or ferments, which carry out 

 many chemical changes in the substances deposited in the cells. 



As we have seen, many of these various bodies exist in the 

 cell-sap in a state of solution ; others, however, are deposited in 

 a solid form. "We can explain the presence of the latter in the 

 same way as that of the others ; they are either reserve or waste 

 products, or charged with a definite duty in the work of the cell. 



The cell-contents, varying thus from place to place and from 

 time to time, stand in a ver\- different position from the proto- 

 plasm, which is constant in its presence during the time the cell is 

 living. The protoplasm carries out the vital work, aided by the 

 various plastids already alluded to, when these are present. The 

 other cell-contents are to be regarded only as aiding or resulting 

 from the work of the protoplasm or the plastids. 



The bodies included in the cell-contents, using the term in 

 this restricted sense, may be divided into two groups : those 

 soluble and those insoluble in the cell-sap. Among the former 

 are included constructive materials for the protoplasm, such as 

 freshly absorbed inorganic salts, and more highly elaborated 

 bodies destined either for immediate use, or temporary storage, 

 or possibly merely on their way through the cell. Various 

 carbohydrates, chiefly 'sugars, vegetable acids and their salts, 

 nitrogen-containing bodies in the form of amides, belong to this 

 category. Other substances, possibly nutritive, possiblj' only 

 the bye-products of nutrition, such as tannin, -^ arious glucosides. 



