80 NATURE TEACHING 



It may seem at first sight wasteful on the part of the 

 plant to make starch, then change it into sugar, and 

 often change this sugar back again into starch in some 

 other part of the plant. We shall, however, see that 

 there is a reason for this, inasmuch as only substances 

 actually dissolved can be carried about from one part 

 of a plant to another ; and that, as starch is insoluble, 

 it has to be changed into a soluble substance (sugar) 

 to enable it to be moved from the leaves where 

 it is formed to other parts of the plant where it is 

 needed. 



A plant requires for its complete nourishment other 

 food - substances besides carbon dioxide and water. 

 These foods are mainly nitrogen and mineral matters. 

 They are usually obtained from the soil, being taken up, 

 dissolved in water by the roots. This watery fluid 

 which permeates the plant is known as the sap, and is 

 in constant circulation owing to the evaporation which 

 we now know goes on from the leaves. As the result of 

 this circulation the mineral bodies taken up in the sap 

 by the root are carried all over the plant, and, combin- 

 ing with the substances formed in the leaves, are 

 enabled to play their proper part in the nourishment 

 and growth of the plant. 



Thus we see the leaf is one of the most important 

 organs of the plant. By their leaves plants breathe, and 

 also obtain a large amount .of their food. The transpira- 

 tion from the leaves maintains the circulation of the 

 sap, thereby ensuring fresh absorption of mineral matters 

 by the root, 



