252 THE BIOLOGY OF THE SEASONS 



shine forth. They are transfigured in the very article of 

 death, in the low beams of the Autumn sun. 



One of the most interesting interpretations of the yellow 

 colour of many leaves in Autumn (and also of the pale 

 yellow etiolation of newly unfolded leaves in Spring) 

 is in terms of economy of material. Pure green chloro- 

 phyll contains carbon, hydrogen, oxygen, nitrogen, and 

 magnesium ; the yellow colouring-matters contain only 

 carbon, hydrogen, and oxygen. Thus, " by keeping back 

 the green chlorophyll in the Spring and reabsorbing it in 

 the Autumn, a saving would be effected in nitrogen and 

 magnesium, which are of great value to the plant." 



Professor Stahl in particular has worked with this 

 interesting theory, for which there is considerable evidence. 

 When a leaf just about to turn yellow is taken from the 

 plant and kept in a moist chamber it remains green, while 

 its neighbour on the plant has changed to yellow. There 

 seems to be a migration of the green chlorophyll. When 

 the veins connecting a corner of leaf with the midrib are 

 severed, the corner remains green, while the other parts 

 remaining in connection with the main conducting vessels 

 change to yellow. Chemical analysis has shown that in 

 the yellow leaf, as compared with the green leaf, there is a 

 reduction in the amount of potassium, nitrogen, phosphoric 

 acid, iron, chlorine, and silica. 



Sometimes it is the double green pigment the chloro- 

 phyll of the leaf that breaks up, forming little heaps of 

 yellow grains, and the leaves are golden. Sometimes, on 

 the other hand, amid the flux of molecules in the dying 

 leaf, there appears a special decomposition product 

 anthocyanin, which along with the acids so often present 

 stains the leaf with red, or without the acids gives us 

 bluish purple, or along with the yellow grains above 

 mentioned shines out in gorgeous orange. It goes without 



