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 event, being withdrawn from the cells in 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 phloem) 

 and it is mainly through the cells of the 

 latter that the sugar travels, diffusing 

 from one cell to another. The cells of the 

 phloem are of various shapes, but they are 

 mostly elongated in the direction of the 



