288 



NATURE-STUDY REVIEW 



[16:7— Oot., 1920 



air and from the sap containing food from the soil; the finished 

 product is largely starch. Thus, it is well, when we begin the study 

 of the tree, to notice that the leaves are so arranged as to gain all 

 the sunlight possible, for without sunlight the starch factories 

 would be obliged to "shut down." It has been estimated that on 

 a mature maple of vigorous growth there is exposed to the sun 



nearly a half acre 

 of leaf surface. 

 Our tree appears 

 to us in a new 

 phase when we 

 think of it as a 

 starch factory 

 covering half an 

 acre. 



Starch is plant 

 food in a con- 

 venient form for 

 storage, and it is 

 stored in sap- 

 wood of the 

 limbs, the 

 branches and 

 trunk, to be used 

 for the growth 

 of the next year's 

 leaves. But 

 starch cannot be 

 assimilated by 

 plants in this 

 form, it must be changed to sugar before it may be used to build 

 up the plant tissues. So the leaves are obliged to perform the 

 office of stomach and digest the food they have made for the 

 tree's use. In the mysterious laboratory of the leaf-cells, the 

 starch is changed to sugar; and nitrogen, sulphur, phosphorus 

 and other substances are taken from the sap and starch added 

 to them, and thus are made the proteids which form another part 

 of the tree's diet. It is interesting to note that while the starch 

 factories can operate only in the sunlight, the leaves can digest 

 the food and it can be transported and used in the growing 



A stump showing rings of growth. 



