THE GREEN LEAF 67 
purposes. Sugar then begins to form in the 
manufacturing cells. But it is a character- 
istic feature of this, as of so many other 
chemical reactions, whether in the living cell 
or in a test-tube, that the rate of formation 
of the soluble product slows down as the con- 
centration of that product increases. Any 
such wasteful lowering of the rate of produc- 
tion is avoided in the plant cell by the 
starting of a second process, whereby insolu- 
ble starch is formed as soon as the con- 
centration of the sugar in the cell reaches a 
certain point. The sugar is thus continually 
prevented from accumulating in quantities 
sufficient to bring about the cessation of 
photosynthetic activity within the cell. 
As long as the leaf remains attached to the 
tree, a certain amount of the sugar is, in any 
event, being withdrawn from the cells in 
which it is being manufactured. This sugar 
does not, however, diffuse from cell to cell 
in any casual direction. Thus it does not 
readily pass from one palisade cell to its 
adjacent neighbour. But it does very readily 
pass into the subjacent cells, and through 
them to the vascular strands of the leaf. 
These strands consist, as already explained, 
of wood and bast (or xylem and phloém) 
and it is mainly through the cells of the 
latter that the sugar travels, diffusing 
from one cell to another. The cells of the 
phloém are of various shapes, but they are 
mostly elongated in the direction of the 
