164 A TEXTBOOK OF GENERAL BOTANY 
increases with the length of the daily exposure. When plants 
which germinate in the spring and flower in the fall are forced 
into bloom early in the summer by shortening the daily expo- 
sure to light, they are small at the time of flowering. 
In the tropics the days are always shorter than the summer 
days of temperate zones. Plants from the tropics may fail to 
flower during the long days of northern summers and produce 
: flowers and fruits abun- 
dantly in greenhouses 
_ during the shorter days 
of winter. 
The fact that the 
days are shorter in 
the tropics than they 
are during the summer 
in temperate zones may 
the plants of the tem- 
perate zone do not suc- 
ceed in the tropics. 
Fic. 161. Section of a gall on a Cissus root in Abnormal forms of 
which a bud of Rafflesia manillanais developing growth. Stems and 

Later the bud will burst through the gall. Com- 
pare with Fig. 7 quently show abnor- 
mal growths. One of 
the most common is a flattened form, called fasciation, which 
occurs when a stem has several growing points instead of a 
single one. This abnormality may be inherited, as in the cocks- 
comb (Fig. 160), or it may be due to an injury, as by insects. 
Galls are another common class of abnormality. They may be 
produced by parasitic bacteria, fungi, flowering plants (Fig. 161), 
or various classes of animals, especially insects. Insect galls 
are very numerous, those produced by a given insect on the 
same kind of plant being constant in form. The mother insect 
lays. its eggs in the host plant; and after the eggs hatch, the 
tissues of the plant proliferate and produce the galls. 
explain why many of | 
other plant organs fre-. 

