HOW STARCH IS STORED IN PLAXTS. 261 



To produce this acid then, the vines may absoro carbonic 

 acid from the atmosphere, combine it with the water of the 

 sap and throw off into the sunshine the residue of oxy- 

 gen. And as we know that all these processes do go on in 

 plants, it is reasonable to assume that this result is due to 

 them. 



By another change which is quite as simple, but not nec- 

 essary to explain, grape sugar or the sugar of fruits may be 

 changed into tartaric acid by the absorption of oxygen and 

 the escape of water. The malic acid of the apple, and the 

 pear, and other fruits, may be formed in precisely the same 

 ways; and it differs from the former acid only in having 

 one equivalent less of oxygen in its composition. 



When the seed is ripe the functions of the animal plants 

 of which the common farm crops consist, are discharged. 

 The absorption and decomposition of carbonic acid by the 

 leaves, and the supply of nutriment to these are no longer 

 required, for their growth is completed. The leaves there- 

 fore begin to absorb oxygen, and decompose; lose their 

 green color and turn yellow ; and prepare to return to their 

 original elementary substances of which they were at first 

 compounded. 



Perennial plants however have a further function to per- 

 form. A supply of food has been deposited in the seeds for 

 the sustenance of the germs which may spring from them, 

 and in the buds which have been formed to begin the growth 

 of another year. When the leaves have ripened and wither 

 and fall, the sap which has circulated through them is con- 

 verted into woody fiber and starch. In some plants this 

 starch is stored up in the stems, as in the potato; the tubers 

 of which are but thickened stems and the eyes merely buds; 

 the starch being intended for the nutrition of the buds when 

 they shall start into growth to renew the plants. The woody 

 fiber of trees is deposited between the bark and the wood of 

 the stem, to form the annual layer by which the tree in- 

 creases in bulk. This layer cf new wood however is depos- 

 ited only under the bark and around the stem and conse- 

 quently the stem increases only in thickness, and never in 



