THE METABOLISM OF PLANTS. 165 



travels from the leaves we should expect to find > peptone 

 in leaves more commonly and in larger quantity than is 

 actually the case. On the other hand we have seen that 

 crystallisable nitrogenous organic substances, amides, have 

 been found in the leaves of various plants and in considerable 

 quantity. We may infer that the proteid which is formed in 

 the leaf is decomposed, and that these amides are some of the 

 products of the decomposition. These substances diffuse 

 readily, and, if we neglect the possibility of the direct conduc- 

 tion of proteid by means of the communicating protoplasm of 

 adjoining cells, it is in the form of these substances that nitro- 

 genous organic substance is distributed throughout the plant. 

 Without entering at present into detail regarding the 

 chemistry of the changes which the organic substances formed 

 in the leaf undergo as a preliminary to their distribution 

 throughout the plant, we will proceed to ascertain what is the 

 mode of their distribution and what are the tissues in which 

 they travel. Their distribution takes place according to the 

 same principle as that of the substances absorbed from with- 

 out by the roots : they travel to those parts of the plant in 

 which a chemical alteration of them is going on, either to the 

 growing-points, at which new branches and leaves are being 

 formed and material is required for the formation of proto- 

 plasm and cell-wall, or to organs which serve as depositories 

 of reserve-materials such as buds, bulbs, roots, tubers, etc., in 

 which both nitrogenous and non-nitrogenous substances are 

 being stored up for the use of the plant at the commencement 

 of its next period of growth, or seeds or spores, in which 

 similar provision is being made for the nutrition of the young 

 plant to be developed from them during the early stages of 

 its growth. They travel either in the parenchymatous tissues 

 by osmosis from cell to cell ; or in continuous vessels the 

 sieve-tubes and the laticiferoiis cells and vessels, by diffusion, 

 their movement in these vessels being promoted by the sway- 

 ing to and fro of the subaerial parts of the plant under the 

 influence of the wind. 



It has been observed that the sieve-tubes of many plants (Vine, Lime, 

 Poplar, Walnut) cease to be continuous vessels during the winter in con- 



