ASSIMILATION OF CARBON 



199 



quently darkened for some days. A pretty experiment may 

 be made on a single large leaf of a sunflower. The leaf should 

 be encased in tinfoil and kept flat, a pattern then cut out from 

 the upper face of the tinfoil. For instance, the words " starch" 

 and " light " in capital letters may be cut out. The leaf must be 

 left for some days (still attached to the plant). It is then re- 

 moved and tested for starch, and it will be found that, after 

 treatment with methylated spirits and subsequendy with iodine, 

 the pattern will be marked in dark blue, whereas the rest of 

 the leaf will be yellow in colour. Only those parts of the leaf 

 which have been exposed to the light have manufactured starch 

 (fig. 239). If we use a plant with variegated leaves which 

 have patches of white, after darkening the leaves for several 

 days, and then exposing them to the sunlight for one day, we 

 find that starch is present only in the green parts of the leaf. 

 This again illustrates the fact that only the green parts absorb 

 carbonic acid and build up starch by its aid. If we kill the 

 leaves they will not manufacture starch. 



How does the carbonic acid get inside the leaves? If 

 we fit a leaf into an air-tight india-rubber stopper, and place 

 it in connection with the apparatus as 

 given in figure 240, and we then suck 

 at the tube, the suction drains away 

 the air from above the water in the 

 bottle, and we see bubbles of air com- 

 ing from the cut end of the leaf-stalk 

 to the surface of the water. We can 

 continue this experiment for a con- 

 siderable time, thus proving that the 

 air which bubbles from the leaf-stalk 

 is not simply air which was inside the 

 leaf; butthat ithascomefromtheatmos- 

 phere outside the bottle, passing in by ^'s- 240. 



the lamina, and travelling down the leaf-stalk. The leaves of the 

 majority of ordinary plants absorb carbonic acid by their lower 

 surfaces, and only slightly or not at all by their upper faces. 

 Hence a leaf coated with vaseline over its whole surface, or over 

 only its lower face, does not manufacture starch at the expense 

 of carbonic acid. But if only the upper surface of the leaf be 

 painted with vaseline, the carbonic acid can enter by the lower 

 face of the leaf, and starch will appear. 



