712 
CHEMICAL PROCESSES IN THE PLANT. 
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 
occurrence of which does not affect the significance here attributed to the laticiferous 
vessels ; the occurrence of products of decomposition in the blood does not prevent 
us from regarding the blood-vessels as organs which serve to transport nutritious sub- 
stances \ 
The mode of motion of the assimilated substances is usually molecular; z'.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 
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, and we find such a substance 
in the conducting tissues of the stem, namely, glucose. But if this glucose had to 
undergo no further change in the tubers, a solution of glucose of constantly in- 
creasing concentration would be uniformly distributed through the conducting 
pores of the sieves, warrants the view that the sieve-tubes are conducting organs for starch in the 
same sense that they are for albuminoids. These small quantities of starch may pass into the sieve- 
tubes from the neighbouring parenchyma, to be used there, in young organs, as plastic material, or, in 
older organs, to take part in the formation of albuminoids. It may be that these substances are 
formed in the sieve-tubes out of carbo-hydrates and nitrogenous compounds, calcic sulphate being 
decomposed and a formation of crystals of calcic oxalate taking place in the cells surrounding 
the phloem. 
^ Sec also Faivre, Sur le latex du murier blanc ; Ann. d. Sei. Nat. ser. V. t. lo. 
