THE COMMONER METABOLIC PRODUCTS 455 



as storage reservoirs for food-substances or for excrete products. By the 

 removal of the latter, reactions injurious to the protoplasm may be pre- 

 vented (cf. Sects. 7, 19, 22, 93). 



The same generalizations apply to all metabolic products, and not 

 merely to plastic substances. Those soluble aplastic substances which 

 remain permanently present in the living cell are apparently for the most 

 part contained in the vacuoles. This localization is especially evident in 

 the case of soluble pigments, which, like many other substances, may be 

 restricted to particular vacuoles, showing that the latter organs have not 

 always the same functional value. The importance of the excretion of 

 such substances as tannic acid, poisons, &c. into the vacuoles can hardly 

 be over-estimated, for, if present in the plasma, they might kill or fatally 

 injure its living parts. Certain poisonous aniline dyes may, however, 

 when slowly absorbed, accumulate in large quantities in the cell-sap without 

 injuring the protoplasm (Sect. 16). 



Any substance which is subject to metabolic change may be termed 

 plastic, although in some cases the purpose of the metabolism may simply 

 be to render possible the excretion of a particular waste product, such as 

 oxalic acid, for example (Sect. 86). It is, however, always possible that 

 under certain conditions a plastic substance may remain temporarily 

 intact, or even permanently so, if the causes and conditions which 

 regulate metabolism remain such as to render the substance in question 

 unnecessary, and hence to prevent its being drawn into metabolism. 

 Reserve-materials may be protected in this manner, whereas in a starved 

 plant substances may be utilized which, under normal conditions, behave 

 as aplastic material. It often happens that in case of need dormant 

 powers may be aroused which are not exercised under normal conditions, 

 and hence the metabolism of a plant during starvation may be abnormal in 

 many respects. 



Even a starved plant is unable to consume the whole of the plastic 

 material which it contains : thus starch-grains frequently do not disappear 

 from the guard-cells of the stomata, or from the the cells of the root-cap, 

 while dead or dying cells frequently contain considerable amounts of 

 sugar, &c., and hence arises the fact that death may occur before turgor 

 disappears l . Certain plastic substances must remain permanently present 

 in order that life may be maintained, and thus living protoplasm, even 



1 Cf. Stange, Bot. Zeitung, 1892, p. 277; Copeland, Einfluss von Licht u. Temp, auf Turgor, 

 1896. [An irretrievable injury is not necessarily immediately followed by death, and it is indeed 

 impossible to say precisely when the latter occurs. The profound change in the physical properties 

 and permeability of the protoplasm, which finally follows a fatal injury, is usually accepted as the 

 only certain sign of death, and in accordance with this definition it would perhaps be more correct 

 ' to say that not a dead but a dying cell may continue to exhibit turgidity.] 



