446 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



development, as for example occurs in winter, or when seeds ripen. This is 

 simply an especially marked example of the self-governing power over all 

 vital functions which a plant possesses, a power which is indeed absolutely 

 essential to enable an appropriate response to be made to changed external 

 conditions. It is owing to this self- regulatory power that the metabolic 

 activity and formative tendencies in particular cells or in all of them 

 alter as development progresses. Similarly a deficiency or superfluity of 

 nutrient material or of special metabolic products may cause more or less 

 marked acceleration or retardation of single functions or of vital activity in 

 general, while the most various external agencies may produce similar 

 results. Thus after the formation of the cell-wall has ceased, a new 

 production of cellulose may be induced by plasmolyzing the protoplast, and 

 similarly an over-accumulation of sugar causes the cessation of its produc- 

 tion by the assimilation of carbon dioxide (Sect. 54) as well as by ferment 

 action or decomposition, while certain plants, when fed with sugar, cease to 

 form the diastase they previously produced in abundance. Similarly the 

 production of many other substances, such as pigments, poisons, &c., may 

 be temporarily or permanently suppressed, and the white sports of blue 

 flowers, as well as the occurrence of sweet almonds free from amygdalin, 

 show that this loss need not involve any injury to the plant. Just as the 

 plant does not require the universally present sodium compounds, so also 

 may non-essential carbon-compounds be formed in metabolism, though these 

 may, however, be of marked biological importance. Even the production of 

 essential formative and plastic substances may not be continuous, whereas 

 a stoppage of respiration or of katabolism must necessarily disorganize the 

 normal vital activity. 



These and other facts show clearly that under normal conditions the 

 different functions have sub-maximal demands made upon them, and hence 

 not only an increase but also a further diminution in their activity is 

 possible. Any such change may either involve a single function almost 

 entirely, or may affect others as well, and often simultaneously may accelerate 

 some and retard others. It is in this manner that formative metabolism is 

 caused gradually to diminish as growth ceases. Metabolism must be 

 capable of a certain amount of modification in those organisms which can 

 grow upon a variety of nutrient media, for in the main the composition of 

 protoplasm is always the same, although the accessory products it produces 

 may differ markedly. 



There is no doubt that certain carbon-compounds take part in the 

 maintenance of turgidity, and may also act as reserve materials or be of use 

 in other ways. Particular carbon-compounds form an essential part of 

 living protoplasm, but quantitative or qualitative changes may occur in 

 the latter although this has not yet been proved to be the case. If 

 potassium takes part in the constitution of protoplasm, then in those fungi 



