34 A LABORATORY MANUAL OF BOTANY 
have anything to do with the relative time of leaf fall? 
Compare also isolated plants and those closely crowded. 
Make a longitudinal section through the base of a leaf 
which is just ready to fall, and determine why the leaf sepa- 
rates from the twig at that point, and why green leaves 
are so much harder to pull off than these autumn leaves? 
Examine leaves of such plants as buckeye, locust, walnut, 
hickory, and sumach. Do their devices for leaf fall vary in 
any way from the types already studied ? 
Is frost the cause of the development of autumn colors 
in leaves and does it cause the leaves to fall? 
As unfavorable weather approaches, the water-supply of 
plants decreases, and this is supposed to induce the plant 
to remove from the leaf the stored foods and living materials 
within it, at the same time to prepare to remove the leaf in 
order to reduce the amount of surface exposed to evapora- 
tion and cold. While these changes are going on, the varia- 
tions in color appear. Doubtless some color changes are 
due merely to the removal or destruction of the chlorophyll. 
It is known that as chlorophyll appears and disappears a 
yellow coloring-matter is formed. But not all the colors 
can be explained in this way. A comparatively recent sug- 
gestion seems to account for the formation of some red col- 
ors. It is known that the presence of solutions of sugar 
sometimes induces formation of red colors, as has been 
proved by growing water-plants in sugar solutions. It is 
also known that starch is a storage form of food, that it is 
often stored in leaves, and that it must be changed to sugar 
before it can be removed from the leaf. These things 
would account for the presence of sugar solutions, and pos- 
sibly for the development. of red colors in the leaf. But 
there. are many shades of color developed that apparently 
are not accounted for in this way, and no satisfactory ex- 
planation of their formation has as yet been offered. 
