194 PLANT PHYSIOLOGY 



arises: Which come first, the anthocyans or the sugars? The 

 facts favor the sugars. 



3. They absorb heat rays. The red pigments certainly absorb 

 some heat, and Keeble found that the side of a leaf where antho- 

 cyan was abundant was 2° C. higher than where no anthocyan 

 was present. Ewart thinks this absorbing action protects the 

 chlorophyll against too strong light, but the majority of workers 

 seem to think that the purpose is to absorb as much energy as 

 possible. This would be of advantage to alpine and arctic plants 

 and to young developing shoots in the spring. The production of 

 the red coloring matter in autumn leaves and in fruits permits 

 more energy to be absorbed and thus hastens the metabolic 

 processes of ripening which precede the fall of the leaf and fruit. 



4. They attract or repel animals. Believers in such theories 

 come to the rather paradoxical conclusion that animals are repelled 

 by red foliage and roots but are attracted by red fruits. It is 

 always dangerous to try to explain similar appearances by similar 

 causes. In the case of fruits, the bright colors certainly have some 

 survival value. 



5. They result in cross-pollination in flowers. Most insect- 

 pollinated flowers are brightly colored or have strong odors, and, 

 while many investigators have shown that bees and other pol- 

 linating insects are very myopic, being guided much more by the 

 odor than by sight, others have not been so easily convinced that 

 the colors of flowers are unimportant. It seems hard to conceive 

 that such conspicuous objects have not had some survival value 

 in this connection. 



6. They are important osmotic constituents of the cells. The 

 same advantages may be derived in this respect from anthocyans 

 and other soluble pigments as from any soluble compounds. 



Autumnal Coloration.— The cause of the coloration of autumn 

 leaves has already been mentioned. Upon the approach of cold 

 weather the chlorophylls disintegrate and disappear. This per- 

 mits the yellow carotinoids to be more easily seen and, at the same 

 time, if sugars are present, the rich reds and lavenders of the 

 anthyocans appear as they are formed. The browns are due to 

 flavones or, more often, to tannins in the cell walls. Plants rich 

 in sugars like the maples and birches are more likely to be bright 

 red or yellow; while those rich in tannins, e. g., the oaks and 

 beeches, are brown in color. Since the bright yellows and reds 



