48 THE BOOK OF NATURE STUDY 



it is the green colouring matter, the chlorophyll, which makes 

 starch. 



Another experiment that any one can try is to cover up the 

 leaf of a plant with tinfoil for about twenty-four hours and then 

 to test for starch with iodine, as just described. Or plants 

 may be grown in the dark and the iodine test applied. It will 

 invariably be found that when excluded from light, starch is 

 not formed, because the chlorophyll has not been able to do its 

 work. 



The chlorophyll is present in the form of small corpuscles in 

 all the cells of the leaf except in the thin skin or epidermis on each 

 side. The structure of a leaf is seen in Fig. 28. 



FIG. 28. Transverse section through leaf-blade. <?, epidermis ; p, cells 

 containing chlorophyll ; pp, cells with chlorophyll and intercellular 

 spaces ; /!?, lower epidermis ; J, stomate. 



This was drawn under a low power of the microscope, and does 

 not show the grains, but merely the layers of cells, forming the 

 thickness of the leaf. Near the upper surface, two rows of cells 

 may be seen without any spaces between them, whilst the other 

 layers have spaces. It follows that the upper face of the leaf 

 will be darker in colour than the lower, because the chlorophyll is 

 greatest in quantity there. The majority of leaves have a darker 

 upper surface and a lighter under surface owing to this arrange- 

 ment of the chlorophyll. There are some plants, however, with 

 leaves of the same colour throughout, owing to the absence of 

 these two rows of cells without intercellular spaces ; the chloro- 

 phyll is evenly distributed, and the leaf is yellowish rather than 

 green, as in the Stonecrop. 



