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 



Fig. 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 

 Eire 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 



