188 PRACTICAL COURSE IN BOTANY 



plant suffers, not from the direct effects of cold, but from 

 the warmth preceding it, which stimulated the transforma- 

 tion into starch of the sugar that would have prevented the 

 loss of proteins. On the same principle we may account for 

 the puzzling fact that the sunny southern side of trees and 

 shrubs usually suffers more from the effects of sudden frost 

 than the shaded and colder northern face. 



In apparent conflict with this reasoning is the fact that 

 sugar cane and the sugar beet are peculiarly susceptible to 

 cold. This, however, does not invalidate the premises es- 

 tablished by Lidforss's researches, but merely emphasizes 

 the need of further investigation, which may either reconcile 

 all the facts, or modify their interpretation. 



205. The colors of autumn leaves. These are due to 

 the breaking up and disappearance of the chlorophyll when 

 the leaf factory has to " shut down " for want of raw ma- 

 terial to work with (203). It is closely connected with the 

 appearance of frost, since the same changes of temperature 

 which produce frost cause the cessation of sap flow that 

 brings about the disorganization of the chlorophyll and the 

 formation of various pigments derived from it. Besides 

 these, leaves may contain other coloring matters that are 

 perceptible only when the chlorophyll disappears; and in 

 the sap there is a reddish pigment which becomes either a 

 very bright red, or a dark purplish maroon, from the effect 

 of chemicals that combine with it in the leaves. With these 

 coloring materials at command it is easy to see how the 

 autumn woods can assume such splendid hues. 



Practical Questions 



1. How would you explain the fact that the outer twigs of trees g 

 are the most leafy? (99, 194; Exps. 57, 74.) 



2. Is the common sunflower a compass plant ? Is cotton ? 



3. Are there any such plants in your neighborhood ? 



4. Compare the leaves of half a dozen shade-loving plants of you 

 borhood with those of as many sun-loving ones ; which, as a genen i r it ing, 

 are the larger and less incised ? 



