8E0T. 11 PHYSIOLOGY 239 



end of the cut ; thus the substances cannot pass by means of the wood. Whether 

 the sieve-tubes also conduct substances with large molecules, such as proteids, 

 without their previous decomposition, is still doubtful. 



C. Eegeneration of Keserve Materials 



Sooner or later the reserve materials mobilised by the help of 

 enzymes are again converted into substances with large molecules. 

 This occurs at any rate at the end of their transport, whether they 

 are again deposited as reserve materials or are employed as 

 constructive substances. Thus, for example, glucose formed in a leaf 

 may pass to a seed or a tuber and be there transformed into starch or 

 cell-wall. When the transport is for a considerable distance the 

 formation of reserve material may go on by the way and not only at 

 the end of the journey. This is specially well seen in the case of 

 starch. Along the routes of sugar-transport so-called transitory starch 

 may be formed in every cell. This starch formation diminishes the 

 concentration of the solution, and thus helps to maintain the continued 

 motion of the diffusion current. 



D. Other Products of Metabolism (^^) 



The chemical activity of the vegetable cell is by no means limited to the 

 production of the substances mentioned ; the number of chemical compounds 

 found to be derived from the first product of assimilation is a matter of continual 

 surprise. Of most of them neither tlie 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, while they are produced by some Fungi to acidify the medium 

 in which they live and render it less favoui'able for competing organisms. The 

 conditions of formation and the significance of the equally widely-spread tannins 

 are quite unknown. The function of the glucosides is also imperfectly under- 

 stood. These are compounds of sugars with a number of different substances. 

 They are soluble in water, and by the action of ferments or dilute acids are 

 broken up into glucose and other derivative products. It is conceivable that 

 the formation of glucosides (and tannins) serves to locally fix substances 

 which otherwise would readily diosmose. It might even be suggested tliat 

 the polysaccharides, starch and cellulose, might be regarded as glucosides of 

 the sugars themselves. In the Amygdalaceae (Bitter Almonds, Plum seeds), 

 AMYGDALiN is found wliich on fermentation yields hydrocyanic acid in addition to 

 benzaldehyde. The Lima or Java beans {Phaseolus lunatus), which of late years 

 have been used for feeding cattle, also liberate hydrocyanic acid from a glucoside. 

 In the wild form this may be in such quantity (0"16 per cent) that fatal poisoning 

 has resulted. From a similar cause Sorghum, Glyceria, and Lotus may be more or less 

 poisonous. Glucosides appear in the Solanaceae as the poisonous solanin, in the 

 Cruciferae (mustard seeds) as myronic acid, in the bark of the Horse-chestnut as the 

 extremely fluorescent aesculin, in species oi Digitalis as the poisonous digitalin, 

 and in the cortex of Willow as "ALICin. Certain plants {Indigofera, Polygonum 

 tindorium) contain indican, the glucoside of indoxyl ; the latter substance is con- 



