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 generally 
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 your neigh- 
borhood with those of as many sun-loving ones; which, as a general thing, 
are the larger and less incised ? 
