I 



CHAPTER II. — SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 725 



or a starchy tuber like the potato, germinates, the starch-grains 

 are gradually dissolved, the starch being converted into maltose. 

 When the tuber of the Dahlia or Artichoke, which contains inulin 

 as the non-nitrogenous reserve material, germinates, the inulin 

 disappears and is gradually replaced by grape-sugar. When an 

 oily seed germinates, the oil-drops become less and less apparent, 

 as the oil is gradually decomposed by ferment-action into glycerin 

 and fatty acids ; the next step is the formation of carbohydrate 

 (sugar or starch), probably from the products of the decomposition^ 

 of the oil, a process which involves the absorption and fixation of 

 oxygen, since carbohydrates contain a higher percentage of oxygen 

 than does any form of fat or oil ; and then, finally, any starch 

 so formed is converted into sugar. Similarly, the aleuron-grains 

 in a germinating seed gradually disappear, the indiffusible proteids 

 'composing them being decomposed by the action of a proteolytic 

 enzyme into peptone, and then into amides, in which form they 

 are conveyed osmotically to the growing embryo. Finally, it is 

 obvious that the indiffusible proteids which are conveyed from 

 part to part in the sieve-tissue of vascular plants (see p. 705) 

 must eventually be distributed osmotically in the form of dif- 

 fusible compounds, probably amides, to the adjacent parenchy- 

 matous tissues, and it is probable, though not yet ascertained, that 

 here again a proteolytic enzyme is involved. 

 J It is worthy of note that the chemical decompositions effected 

 \ by the agency of the enzymes do not i n volv e either an absorption 

 1 of oxygen or an evolution of carbon dioxide, whe^as this is the 

 ' case in the decompositions effected by the agency of living proto- 

 plasm. But to the latter general rule there are some exceptions. 

 Thus, it has been observed that succulent plants, such as Cactaceoe, 

 Crassulaceae, etc., absorb free oxygen without exhaling any carbon 

 dioxide ; but it has further been found that, under these circum- 

 stances, there is an accumulation of organic acids (especially 

 malic) in the tissues. The explanation is this: that, owing to 

 some peculiarity in their catabolic process — possibly to the im- 

 perfect aeration of their tissues — instead of producing the very 

 simple substance carbonic acid, which would be exhaled in the 

 form of the gas CO,, these plants produce more complex, less 

 highly-oxidised carbon-acids, which are not volatile at ordinary 

 temperatures, and which therefore remain in the cells of the 

 plant : these acids (at least malic acid) subsequently undergo 

 decomposition under the influence of light with the formation of 



