ASSIMILATION AND METASTASIS. 6ot- 



as a centre of attraction ; the molecules stream to the parts where they are wanted 

 because the molecular equilibrium of the solution is disturbed by its consumption. 

 On the other hand every cell which produces a new soluble compound acts on 

 the dissolved molecules as a centre of repulsion, because the continually increasing: 

 concentration occasions at the point of production a streaming of the molecules 

 away from it towards the point of less concentration, the concentration continually 

 decreasing towards the points where the substances are consumed. When the move 

 ment of diffusion is caused by the production and consumption of definite compounds 

 of this nature, the proximate cause of the molecular movement of the dissolved sub- 

 stances must be the chemical processes involved in their metamorphoses. These 

 metamorphoses take place, as we have seen, not only at the points where the sub- 

 stances are consumed in the process of growth, but also in the conducting tissues ; 

 and this production of transitory compounds must therefore favour movement 

 towards the points of deposition and of growth. The formation of insoluble 

 starch is in this sense a fact of peculiar importance. If for instance the starch 

 produced in the leaves of the potato is required to be transported to the tubers, 

 it must necessarily be conveyed in a soluble form, such as we find in the con- 

 ducting tissues of the stem in the form of glucose. But if this glucose had to 

 undergo no further change in the tubers, a soluuon of glucose of constantly in- 

 creasing concentration would be uniformly distributed through the conducting 

 tissues and the tubers ; and the accumulation of the whole of the reserve-material 

 in the tubers w^ould be impossible. The glucose is used up in the tubers in the 

 formation of starch-grains ; and a fresh quantity therefore continually streams in 

 that direction; the whole mass of the material produced in the leaves is therefore 

 gradually transferred to the reservoirs of reserve-material. The starch is first 

 transformed into glucose, and then back into starch ; and it is in this chemical 

 process that the vehicle for the movement consists. Starch is even produced tem- 

 porarily in the conducting parenchyma, but of course cannot be transported as such 

 from cell to cell ; its movement being effected by the grains being dissolved in 

 one cell, the product of solution diffusing into the adjoining cell, and being 

 there employed in the formation of starch-grains which are then again dissolved, 

 and so on. When again cane-sugar is formed in the tuberous roots of the beet, 

 the movement towards the root of the glucose which is produced from the starch 

 assimilated in the chlorophyll is brought about in this way,— every particle of glu- 

 cose undergoes chemical transformation when it reaches the root, and the mole- 

 cular equilibrium of the solution of glucose is thus disturbed ; the root acting like 

 a centre of attraction on the glucose in the leaf-stalks. But the continual form- 

 ation of the solution of glucose in the leaves at the expense of the starch 

 causes in them an increase of concentration and a streaming of molecules towards 

 the root, where the concentration of the solution of glucose is continually decreasing, 

 while that of the solution of cane-sugar increases. The same is evidently the 

 interpretation of the formation of inuline in the tuberous roots of the dahlia and 

 the tubers of the artichoke, and of that of oil in ripening seeds at the expense of 

 the sugar which is conveyed to them. 



The co-operation in the movement towards the parts where the substances are 

 consumed of the tissues of the pressure exercised on the cell-sap by the tension 



