34 



THE BOOK OF NATURE STUDY 



dons of the Radish first show above the surface they are yellow ; 

 so too are the plumules of the pea and bean. Then as the bud 

 unfolds, a tinge of green appears ; still, the prevailing tone is 

 yellow rather than green for some days. Gradually the green 

 deepens, and with the soft, tender green of the young stems and 

 young leaves at the tips of the twigs, forming a refreshing contrast 

 to the darker stems of the preceding year's growth, spring has 

 come. It is a matter of experience that in some years spring is 

 earlier than in others, and in those years there has generally been 



FIG. 17. Bean plant grown 

 in the dark. 



FlG. 1 8. Bean plant grown in 

 the light. 



more sunlight. Putting these facts together, it seems clear that 

 light is essential for the development of the green colouring matter 

 of the leaf. It is easy to verify this by growing some seeds in the 

 dark, and others of the same species in the light. Those grown in 

 the dark produce cotyledons and foliage-leaves of very much the 

 same yellow tint as the leaves of the buds of trees when they first 

 begin to unfold, or of seedlings when they first come above the soil. 

 The difference in the colour of the leaf is due to the absence of 

 light. This affects not only the colour of the leaf, but that of the 



