634 CHEMICAL PROCESSES IN THE PLANT. 



reserve-material ; all the serviceable substances contained in the leaves become in- 

 corporated in the permanent organs. The leaves change colour ; a small quantity 

 of very small shining yellow granules usually remain behind in the cells of the 

 mesophyll as a residue of the absorbed chlorophyll-grains ; and the leaves which 

 ^re emptied in the autumn are therefore yellow. If they are red this is in con- 

 sequence of a red sap which fills the cells in addition to the chlorophyll-grains\ 

 Enormous quantities of crystals of calcium oxalate often remain behind in the 

 deciduous leaves ; the constituents of the ash which are serviceable to the plant, 

 especially phosphoric acid and potassa, are conveyed with the starch and the proto- 

 plasmic structures to the permanent parts ; so that the falHng leaves thus consist 

 only of a skeleton of cell-walls and of the subsidiary products of metastasis which 

 are of no value to the plant. 



The direction of the Transport of the assimilated substances in the plant is 

 determined by the fact that it must take place from the assimilating organs to 

 the growing parts and to the reservoirs of reserve-material \ while at the com- 

 mencement of every new period of vegetation its direction must be from these 

 reservoirs to the growing organs ; and since new organs are usually formed above 

 as well as below these reservoirs and the assimilating leaves, it is obvious that 

 the movements of the assimilated substances must take place at the same time in 

 opposite directions. 



The Cojiduciifig Tissue for the transport of the formative materials consists, 

 in plants with differentiated systems of tissue, of the parenchyma and the thin- 

 walled cells of the phloem of the fibro-vascular bundles. By the parenchyma of 

 the fundamental tissue, which always has an acid reaction, are conveyed the 

 carbo-hydrates and oils; by the soft bast the mucilaginous albuminoids which 

 have an alkaline reaction. Only when the conduction is very rapid, as when the 

 leaves are emptied in autumn, and in plants with very rapid growth (as the castor-oil 

 plant and gourd) are small quantities of starch found also in the sieve-tubes. Where 

 there are laticiferous vessels, they furnish an open communication between all the 

 organs of the plant; they contain albuminoids, carbo-hydrates, and oils, as well as 

 the secondary products of metastasis, as caoutchouc and poisonous substances. 



The mode of motion of the assimilated substances is usually molecular ; /. e. 

 it is a movement of diffusion, especially where the transport takes place through 

 closed cells. The pressure caused by the tension and turgescence of the tissues 

 has in addition a tendency to propel the fluids in the direction of least resistance, 

 which is also that in which they are consumed. In the system of communicating 

 sieve-tubes and laticiferous vessels the movement of the substances is necessarily 

 one of the entire mass, caused by inequalities of pressure, and by the distortions 

 and curvatures which the wind produces. 



As far as concerns the movements of diffusion, it is a general rule that 

 every cell which decomposes any substance, renders it insoluble, or uses it for its 

 growth, acts upon the dissolved molecules of this substance in the neighbourhood 



^ [On the colouring matter of th^ leaves in autumn, see Sorby, Quart, Journ. of Science 1S7] 

 p. 64; and 1873, p. 215.— Ed.] 



