SECT, ii PHYSIOLOGY 225 



that time attained their greatest expansion and efficiency, the surplus 

 of reserve material is greatest at the close of the season, and is 

 stored in special RESERVOIRS OF RESERVE MATERIAL. The growth of 

 the shoots and leaves of the next season and similarly the growth 

 of seedling plants is dependent on such reserve materials. Reserve 

 materials will accordingly be found stored in different forms in the 

 cells of the embryo, or in the surrounding tissues of the seed, in 

 underground rhizomes, tubers, bulbs, and roots, or in the cortical 

 layers, the medullary rays, the wood parenchyma (especially the 

 thin-walled fibres), and the medulla of persistent stems. Conveyed 

 to these depositories of reserve material, the glucose and maltose are 

 again converted into other carbohydrates, usually starch. In other 

 cases the reserve carbohydrates take the form of cane-sugar (the 

 sugar-beet contains 5-8 per cent, and selected varieties 1 8 per cent, and 

 in some cases even 21 and 26 per cent), inulin (Compositae, Cam- 

 panulaceae) or reserve cellulose (e.g. vegetable ivory in the fruit of 

 Phytelephas). Still more remarkable is the transformation of carbo- 

 hydrates into fats and oils, occurring in the ripe and ripening seeds 

 of many plants, in fruits (Olive, oil-palm), and also in strictly vege- 

 tative tissues. In winter the starch in the wood of many trees also 

 becomes converted into oil, but in the succeeding spring it is again 

 changed to starch. It is finally, at the opening of the buds, converted 

 into glucose or maltose, and conveyed by the transpiration current 

 to the young shoots. Other receptacles of reserve material contain 

 scarcely any carbohydrate, but on the other hand there is much more 

 albuminous matter in the form of thick protoplasm, aleurone grains, 

 protein crystals, and fats (seeds of Ricinus). That in germination 

 similar tissues with protoplasm, nucleus, cell walls, etc., are formed 

 from these different materials, seems to indicate that all these 

 constructive materials are of almost equal value to the plants. This 

 is due to the fact that plants can apparently without difficulty 

 transform the carbohydrates, fats, or albuminous substances one into 

 the other, a result not yet accomplished by chemical processes. 



Other Products of Metabolism 



The chemical activity of the vegetable cell is by no means exhausted in the 

 production of the substances mentioned : the increasing number of chemical com- 

 pounds found to be derived from the first product of assimilation is a matter of con- 

 tinual surprise. Of most of them neither the manner of their formation nor their 

 full importance in metabolism is understood. The conditions are not even fully 

 known which are necessary for the formation and functional activity of the OIUJAXK; 

 ACIDS (malic, tartaric, citric, etc.) which may in part be considered as products of 

 imperfect respiration while they are produced by some Fungi to acidify the medium 

 in which they live and render it less favourable for competing organisms. The 

 same holds for the equally widely-spread tannins. The function of the GLUCOSIDKS 

 is also imperfectly understood. These are compounds of sugars with a number of 

 different substances. They are soluble in water, and by the action of ferments or 



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