36 STRUCTUKAL BOTANY 



have been deposited one after the other, so that the 

 innermost layers are the oldest and the outermost the 

 youngest. The oldest part of the granule round which 

 all the layers have been formed is called the hilum. It 

 does not necessarily lie at the centre, for sometimes 

 the layers are deposited very unequally, so that in 

 an old granule the hilum is in a very eccentric 

 position. This is always the case in potato starch. 



In a green plant starch is formed in two quite 

 different ways. First it is formed in the chlorophyll 

 granules under the influence of sunlight. It is easy 

 to demonstrate the presence of small starch granules 

 in the chlorophyll-corpuscles of a Wallflower leaf, after 

 exposure to light. These starch granules are the 

 product of carbon-assimilation that is to say, they 

 are formed by the chlorophyll-corpuscles in sunlight as 

 the result, though not the direct result, of the decom- 

 position of carbon dioxide and water, and the re- 

 arrangement of their elements. 



But starch is also formed in the deep-seated tissues 

 of the stem and the root, i.e. in parts which receive 

 little or no light, and in which there is no chlorophyll. 

 This starch cannot be the product of assimilation, for 

 it is formed under conditions which render assimilation 

 impossible. 



What happens is this : the starch formed in the 

 chlorophyll granules is converted into sugar, 1 and in 

 this soluble form passes down into the stem and root. 

 When it has reached the cells in which starch is 

 to be deposited, the sugar is taken up by certain 

 protoplasmic bodies, which are essentially similar to 



1 See. however, p. 213. 



