
162 A TEXTBOOK OF GENERAL BOTANY 
The advantages of chilling for trees of the temperate zone seem 
to be connected with changes in the character of the stored food. 
During the summer and autumn food is stored in the form of 
starch, which is insoluble. Before this can be used for growth 
it must be changed to sugar, which is soluble, and in these plants 
this change appears to take place most readily at low temperatures. 

Fig. 159. Effect of lengths of day on different types of plants 
Left, Helianthus angustifolius, a ‘t short-day plant.’? Both specimens planted 
March 2 and photographed July 31. Plant at left kept in the dark during a 
portion of each day and exposed to light for only ten hours. Plant at right 
exposed to light for full length of day. Right, red clover, a ‘* long-day plant,’ 
photographed June 28. Plants at left kept in the dark during a portion of each 
day and exposed to light for only ten hours. They assumed a prostrate form of 
growth and were greatly delayed in flowering. Plants at right exposed to light 
for full length of day during spring and early summer. (From work of Garner 
and Allard, Bureau of Plant Industry, United States Department of Agriculture) 
Trees of the temperate zone are afforded valuable protection 
by the fact that in the fall they enter into a condition of dor- 
mancy from which they do not emerge until exposed to prolonged 
chillmg. If such plants were so constituted as to start into 
growth as easily in the warm days of late fall as they do in 
early spring, many species would come into flower and leaf in 
warm autumn days which follow cold ones, and the stored food 
that they require for their normal vigorous growth in the fol- 
lowing spring would be wasted in the growth of new twigs 
