Translocation in Plant Tissues . 
305 
are expressed by specific chemical activities. Cell-walls may become 
thickened, either permanently or for the purpose of temporary storage. 
Production of aromatic compounds may occur in certain types of cells, or 
a glucoside may be stored for a time in one region, while the enzyme com- 
patible with it is confined to another. 
There is also the possibility of change with senescence. 
Benedict 1 studied the leaves of various deciduous plants and concluded 
that the observed facts could best be accounted for by assuming that among 
other changes a decrease in the permeability of the protoplasm (to water 
and salts) occurred with advancing age. 
Again, the inability of many unicellular organisms to continue purely 
vegetative reproduction indefinitely has led to the suggestion that sexual 
fusion in some way rejuvenates the protoplasm. 
It would not therefore be surprising to find that certain protoplasts 
could effect a greater adsorption of sugar than others, so that a higher 
concentration would be reached in their vacuoles before the protoplasm 
became adsorptively saturated. (Cf. Fig. 1.) 
In such cells sugar would tend to accumulate more than in other cells, 
and in this manner a directive influence could be exerted upon sugar 
transference. For example, if the protoplasm lining the sieve-tube segments 
in the leaf became adsorptively saturated with a lower concentration of 
sugar than that needed to produce this result in the petiole and stem sieve- 
tube segments, then sugar would tend to become concentrated more in the 
latter segments and to be withdrawn from the leaf. 
Such gradients in sugar concentration in sieve-tubes have actually been 
found to occur, in the case of leaves allowed to remain in darkness for a day 
or two before being examined for sugar distribution by the osazone method . 2 
The views here set forth are in harmony with what was previously 
known with regard to the flow of sugar in the plant. 
Experiments made by Schimper , 3 for example, -showed that during 
darkness sugar disappeared from leaves, and travelled from regions of low 
to regions of higher concentration, i. e. apparently against the ordinary laws 
of diffusion. 
It is difficult to account for this on lines other than those indicated 
above. An hypothesis which involves the outward passage of sugar through 
the plasmatic membrane of one cell followed by the penetration of the 
membrane of the next protoplast appears to be more arbitrary and less 
comprehensible than that suggested here. 
The present hypothesis enables one to understand how the varying 
and widespread demands made by the plant upon its sugar supplies are 
met, viz. by the propagation of waves of readjustment of concentration 
2 Mangham (1910, 1910-11, 1911 (1), 1915). 
3 Schimper (1885). 
1 Benedict (1915). 
