THE LEAF 



137 



Apart from chlorophyll there is no assimilation of carbon 

 in the plant. 



This chlorophyll is found in cells in bodies of varied 

 form, in small granules, or in disc-shaped (fig. 40) l 

 or band-shaped bodies (fig. 58). These bodies are 

 called chloroplasts. If we keep a plant in the dark 

 for a time and then examine these chloroplasts under 

 a microscope their structure appears quite uniform 

 (fig. 40 a on the left) ; but if we then set the plant in 

 the light, we find after a certain length of time, 

 occasionally after even a few 

 minutes, that tiny granules 

 have appeared in them (fig. 

 40 a on the right). In some 

 plants these granules increase 

 in size, in time protrude them- 

 selves and continue to grow on 

 the side in contact with the 

 chloroplast (fig. 40 b) . They 

 then have a characteristic 

 stratified appearance, and we 

 recognise them as grains of 

 starch. We need not, how- 

 ever, wait until they develop ; 

 we can detect starch in a 

 granule as small as a pin point by colouring it blue 

 with iodine, a reaction with which we are already 

 familiar. 



Starch grains, then, are formed in the chloroplasts, 

 and continue to grow where they are in contact with 

 chlorophyll. We can easily prove that the formation of 

 starch is connected with the decomposition of carbonic 

 acid. To begin with, no starch is formed in the chloro- 

 plasts when the plant is not supplied with carbonic acid ; 

 nor, in the second place, is there any formation of starch 



1 Fig 40 a. On the left, chloroplasts without starch ; on the right 

 with starch grains inside them, b Single large starch grains. 



FIG. 40. 



