122 THE PHYSIOLOGY OF PLANTS 



The fully-formed chlorophyll bodies grow, and become biscuit 

 or dumb-bell shaped. The narrow portion of the grain is almost 

 colourless, and by becoming entirely constricted the granules 

 increase in number. In this way the leaf attains its green 

 colouration, which is so characteristic of the fully developed 

 organ. By this time the cells will have attained their full 

 size, and the intercellular spaces will have made their appear- 

 ance. Even these must have definite positions to be of use to 

 the plant. In the case of the pallisade cells they occur be- 

 tween the lateral walls, and thus the newly-formed substances 

 cannot pass laterally into other pallisade cells, but must pass 

 out at the bottom of the pallisade cell into the spongy paren- 

 chyma, or into the bundle sheath. 



.§23. What substances does the leaf chiefly form ? 



The first product of assimilation which can be detected in 

 the chlorophyll body is the starch grain, which is the chief 

 representative of the so-called carbo-hydrates. This group of 

 substances, to which sugar, cellulose (cell-wall substance), inulin, 

 dextrine, and gums belong, consists of the elements carbon, 

 hydrogen, and oxygen, the last two substances being combined 

 in the same proportions as they exist in water (H^O). The 

 starch occurs in the form of curiously stratified grains, which 

 can be coloured blue, in some cases red, with iodine. Starch 

 may be looked upon as the solid equivalent of the liquid carbo- 

 hydrates which pass from cell to cell in the form of sugar. 

 When the light shines on the leaf, its transpiration is greatly 

 increased, and its cells lose some of their water ; then we may 

 imagine the starchy substance to be forced out of the thickened 

 {concentrated) cell-sap, and the latter will become more liquid. 

 The starchy substance is in most cases not entirely pure, but is 

 mixed with amylodextrin, and this causes the stratification of 

 the grain. The more this substance is present in the starch 

 the redder will it stain with iodine. 'As soon as darkness 

 sets in, the ferments become more active, and transform the 

 starch into dextrin. This latter is converted into sugar, 

 which passes from cell to cell through the cell-wall, and 

 from the leaf into the stem, where it is reconverted into starch, 



