IX Leaves 175 



only in general terms. The starch which is made in the 

 light cannot pass out of the leaf except into the body of the 

 plant, nor can it pass away as such, for the solid starch 

 grains could never get through the walls of the cells. 

 Before it can pass from the leaf the starch has to be fer- 

 mented, it has to be changed into sugar. The same is true 

 in regard to the starchy parts of the food that we eat ; it is 

 a simple matter to masticate and swallow a potato or 

 banana, but before the starch which these foods contain 

 can be of the slightest use to us it must pass through the 

 walls of the food-canal into our blood. Now the starch as 

 such cannot pass through the walls'of cells. The grains are 

 too large. If must be rendered soluble and diffusible ; in a 

 word — digested. This process begins in the mouth, where 

 a ferment contained in the salivary juice changes the starch 

 into a sugar. A precisely similar digestion (by means of a 

 ferment called diastase) takes place in the leaves, and it is 

 in the form of sugar (glucose) that the starch passes into the 

 stem. 



But by what path ? We have spoken of the epidermis 

 or skin of the leaf above and below, of the loose arrange- 

 ment of cells on the lower side, and of the always greenest 

 palisade tissue in which the essential processes of the cell's 

 life go on. But we have not spoken of one of the most 

 important and characteristic parts — the system of '•' veins," 

 which in most common plants forms an intricate net- 

 work through the substance of the leaf. 



These veins form the most durable part of the leaf, for 

 in the skeleton leaves which we see rotting away in the 

 ditches the veins persist while all else, unless perhaps the 

 firm outermost sheath, has disappeared. There is no 

 doubt that the " veins '^ fonn a sort of supporting skeleton 

 for the leaf. They are the firm beams around which the 

 more delicate substance of the leaf has been built up. But 



