164 LIGHT, VEGETATION AND CHLOROPHYLL 



The Chloroplasts 



In the palisade tissue in the interior of the cytoplasm of 

 green leaves are small, strongly-coloured green plastids called 

 chloroplasts. They are so abundant that they constitute a 

 quarter or even a third of the dry weight; there are from 

 twenty to a hundred in each of the cells of the palisade tissue. 

 With care, they can be extracted intact. See Fig. II, 1 . 



Fig. II, 1. Cross-section of a leaf. 1, Upper epidermis ; 



2, Palisade tissue; 3, Chloroplasts; 4, Lacunose tissue; 



5, Lower epidermis; 6, Stroma 



In the higher plants, they are in the form of a disc of 3 to 

 10 /x in diameter and 1 to 2 /x thick. They are bounded by a 

 semi-permeable membrane. 



The microscope resolves the chloroplast into a certain 

 number of small discs — from ten to a hundred — that are 

 called grains of chlorophyll. The diameter of these grains 

 varies from 0-2 to 2 jn; for example, for spinach they are 

 0-6 /x in diameter and 0-1 ^ thick. All the chlorophyll is located 

 in these grains the framework of which is formed by a 

 support — a colourless stroma. 



