THE LEAF 119 



cells form what is called the pallisade parenchyma. The loose, 

 often horizontally elonuated, tissue of the under side of the leaf 

 is termed the spongy parenchyma. The pallisade cells, on 

 account of the greater number of chlorophyll grains which 

 they contain, and because the incident rays of light pass 

 through the entire length of the cell, and illuminate each 

 chlorophyll grain, are the most efficient assimilating cells. At 

 the same time, as they have only their narrow head end directed 

 towards the light, they are able to protect themselves from an 

 excess of light. 



The product of their activity is recognisable very soon after 

 the light has been acting upon the tissues; it makes its 

 appearance in most plants within the chlorophyll bodies in the 

 form of starch g-rains. In some natural orders {e.g., in the 

 Com2)ositce), in the place of starch a soluble substance, inulin, is 

 formed, which also consists of hydrogen, oxygen, and carbon, 

 in about the same proportions as starch. If the water be 

 withdrawn from cells containing this substance — as will be 

 the case if the tissues are treated with alcohol — the inulin 

 attaches itself to the cell-walls in the form of solid crystalline 

 masses, the so-called sphseroerystals. Certain reactions, too, go 

 to prove that new albuminous (nitrogen-containing) substances 

 are formed within the chlorophyll grain ; so that the two chief 

 groups of the organic matter are represented within the chlo- 

 rophyll granules. 



If the pallisade cell cannot itself make use of the newly- 

 formed organic material, it passes out of the cell during the 

 night in the form of sugar and other soluble substances. 

 These are either absorbed by the adjoining cells of the spongy 

 parenchyma and conducted towards the base of the leaf, or 

 they pass directly into the parenchymatous sheath which 

 surrounds the vascular bundle. These bundle sheaths (Fig. 

 17, Sch) are distinguished, according to their contents, as 

 sugar sheaths or starch sheaths, and form the actual canals 

 along which the assimilated substances pass and rapidly 

 reach the stem. 



