THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. II 163 



(which, however, strictly speaking, should not be referred to here). A more 

 detailed account of the distribution of the different varieties of sugar is as 

 out of place as a complete enumeration of those which have been found. 

 We need only note that cane sugar and inulin undergo alteration during 

 germination, although we might assume a direct conversion from their 

 solubility in water. The alteration once more consists in a hydrolytic decom- 

 position by the agency of enzymes. The enzyme invertase decomposes the cane 

 sugar into equal parts of dextrose and levulose, while inulase transforms 

 the inulin into levulose. 



Proteid is always present among the nitrogenous reserves of perennial plants, 

 occurring occasionally in the form of crystals (e. g. in the potato) ; it, however, 

 never forms aleurone grains owing to the large amount of water present. Besides 

 proteid, amido-compounds, such as asparagin, leucin, and tyrosin, also occur, 

 and these, in the majority of cases, may be assumed not to arise from precedent 

 proteid but to be stored as such. In the tubers of a certain variety of potato, 

 ScHULZE (1882) found 56 per cent, of the total nitrogenous substance present 

 consisted of amido-compounds, and only 44 per cent, of proteid. In the sugar- 

 cane Shore Y (1897) found the simplest amino-acid, glycocoH, a substance which 

 has not as yet been shown to occur elsewhere in plants. 



The most extensive storage tissue occurs in trees, where all the parenchyma 

 cells of the wood, of the cortex, and also of the pith, both in root and stem, are 

 filled with reserves. The central elements of the wood of many trees which 

 gradually pass over into duramen and, on losing their vitality, cease to store 

 reserves, form an exception. Starch is the most generally distributed of all the 

 non-nitrogenous substances in such plants, and, owing to the ease with which it 

 can be recognized, its behaviour can be closely followed. Storage of starch begins 

 in May or June, and generally first of all in the cells of the root ; later, it appears 

 in the stem, and finally the cells of the branches and finer twigs become filled with 

 it. During the winter it undergoes alteration, completely or partially, enabling 

 it to be translocated to other regions ; but in spring, before the buds come out, 

 the same conditions as in autumn are re-established, and in order to render the 

 starch mobile, diastase must be produced in sufficient quantity. [In addition 

 to starch, hemicelluloses also occur as non-nitrogenous reserves in trees ; these 

 bodies take the form of thickenings on the walls of the wood fibres or cortical 

 parenchyma, which become again dissolved in spring (Leclerc,I904 ; Schellen- 

 BERG, 1905).] Our knowledge of the nitrogenous reserves in trees is much less 

 extensive ; in general, they consist of proteids and amides. Proteid in a crys- 

 talline form may be demonstrated in some situations, for example in the bud- 

 scales of certain trees, which act a storage organs in the same way as do 

 bulb-scales, and which contain not only proteid but non-nitrogenous substance 

 as well — mostly in the form of reserve cellulose. 



The last type of storage organ, the foliage leaf, brings us back again to 

 structures with which we are more familiar. We have already studied in detail 

 the synthesis of carbohydrate in these organs and have seen that it is probable 

 that proteid is formed there also — if not exclusively, still in very large quantity. 

 It has also been more than once indicated that fats of value in metabolism are 

 scarcely at all formed in the foliage leaf. [In leaves which last for several years, 

 fats acting as reserves certainly occur, at least in winter, just as in the case of trees 

 (p. 175).] So long as the leaf is growing the products of assimilation are at once 

 made use of, or are so rapidly translocated that an accumulation of them is im- 

 possible. In general, the products of assimilation, or more accurately, the surplus 

 over immediate wants, become reserves in the regions of their formation, but 

 they do not remain in this condition for long ; most commonly, they again 

 become mobile during the night succeeding their construction and migrate out of 

 the leaf. In the case of starch, such periodic formations and dissolutions have 

 been definitely demonstrated. Phenomena such as these, however, suggest 



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