220 GROWTH OF STEMS 
Indirectly light is very essential for growth because of its im- 
portance in the manufacture of plant foods. But directly light 
has little effect, unless it is intense, and then it checks growth. 
That most plants grow faster at night than in day time is well 
known; and, although much of the increase in the rate of growth 
at night is due to the greater humidity of the air, some is due to 
the absence of the inhibitive effect that the sun’s rays have on 
Fie. 200.— Pines growing in the open where their trunks are short and 
much branched. From Bulletin 24, North Carolina Geological and Economic 
Survey. 
growth. In Bacteria, where the protoplasm is not protected by 
pigments, the sun’s rays so inhibit growth that they have an im- 
portant germicidal effect. 
On the other hand, if plants do not have sufficient light, they 
are affected in various ways. For example, when plants are grown 
in the dark, as the Potatoes in Figure 198 illustrate, the stems are 
excessively elongated, the leaves are abnormal, and the plant 
lacks chlorophyll, on which account the plant is said to be etio- 
lated. Even plants grown in the shade, having the light only par- 
tially cut off, are usually taller and more slender than plants 
