127 



regions, has already been considered. Recently attention has also been di- 

 rected to the increased change of color in foliage leaves and its peculiar 

 significance as a protecti\e adaptation has been suggested. MacMillan^ 

 treats of these conditions very fully. lie speaks of "ivarming-up colors" 

 meaning especially the red coloring substances which are more abundantly 

 represented in colder regions. Alpine and arctic plants are more often 

 found with blue or violet blossoms than with yellow ; the ends of the twigs 

 are often reddened. The temperature is somewhat raised by the red coloring 

 matter and the influence of cold somewhat weakened. If one thermometer 

 be covered with a green leaf and another with a purple one, while both are 

 exposed to the sun, in a short time the thermometer protected by the purple 

 leaf shows a rise of 6° to io° of temperature. In the same way he found 

 that a thermometer, stuck in a bunch of violets, shows a higher temperature 

 than one in a bunch of cowslips, after an equal ex])osure to the sun. 



The autumnal coloring may be conceived as a definite reaction of the 

 plant to the lowered temperature. The plant provides warmth for itself in 

 its red coloring matter. On this account so many spring flowers are red 

 and violet and autumn flowers blue or red. 



In warm climates plants often assume peculiarities directly opposite to 

 those of arctic or alpine plants. In tropical plants the storage cells are less 

 strongly developed than in related species from colder regions. The buds 

 are less protected, pubescent coverings more rare on leaves and twigs (with 

 the exception of desert plants). Many winter habits disappear. There are 

 fewer biennials. The warming-up colors recede more and more, while 

 \^hite, yellow and spotted blossoms (Orchids) predominate. 



Nature would develop red coloring matter to prevent loss of the super- 

 fluous light and to transform it into warmth and to use it as a stimulus to 

 growth. 



Wit cannot support this theory of the premeditated utility of the red 

 coloring matter as an apparatus, producing warmth and weakening the 

 light, even if we had such an inclination. If the red coloring matter has 

 once been produced, it wdll be effective in the way given. The idea that the 

 plant can produce it as a protection against cold, when the temperature be- 

 comes lower, is not plausible, because in the hottest summer temperature 

 leaves can be reddened. In the Rosaceae which are rich in tannin (Crataegus, 

 for example), I have been able to produce the red autumnal coloring after 

 a few weeks in the middle of summer by girdling the twigs. The fact that 

 in summer the underside of many leaves, when reversed, becomes red within 

 a few days is universally known. Parasites furnish further instances. On 

 the same cherry tree., for example, the leaves of branches attacked by 

 Exoacus Cerasi turn glowing red, while the healthy ones remain green. In 

 many spot diseases the circular fungus centre appears surrounded by red. 

 Amaryllidaceae, whose leaves die down in summer (Hippeastrum etc.), 

 develop carmine spots and stripes. 



1 MacMillan, Conway, Minnesota Plant Life Saint I'aul, Minnesota, 1899, p. 417. 



