PHYSIOLOGY 205 



the season, and is stored in special reservoirs of reserve material. 

 All growth of the succeeding year, either of the plants themselves or of 

 their embryonic offspring, is dependent upon the existence somewhere of 

 a supply of reserve material, which may be utilised by the plant until 

 the organs of assimilation are developed. Eeserve materials will accord- 

 ingly 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 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, 

 which is formed from them by the activity of the starch-producing 

 leucoplasts. In other cases the reserve carbohydrates take the form 

 of cane-sugar, inulin, or reserve cellulose {e.g. vegetable ivory in the 

 fruit of Phytelephas). Still more remarkable is the transformation of 

 carbohydrates into fats and oils, occurring in the ripe and ripening 

 seeds of many plants, in fruits (Olive), and also in strictly vegetative 

 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 carbo- 

 hydrate, 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 the germination of young plants similar 

 tissues with protoplasm, nucleus, cell wall, 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 carbo- 

 hydrates, 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 organic 

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

 imperfect respiration) and tannins, although both are so frequent in plants. The 

 function of the glitcosides is also imperfectly understood. These are nitrogenous 

 and non-nitrogenous compounds, and are not widely distributed. They are soluble 

 in water, and by the action of ferments are broken up into glucose and other deri- 

 vative products. In the Amygdalaceae they appear as amygdalin, in the S'olaii- 

 accae as the poisonous solanin, in the Cruciferae (mustard seeds) as mykoxic 

 acid, in the bark of the Horse-chestnut as the extremely fluorescent iESClTLiN, in 

 species of Digitalis as the poisonous digitalin. Coniferix, which is contained 



