I46 BOTANY OF THE LIVING PLANT 



„ (lW to be considered, while in addition other factors, not so far 

 mentioned, also affed growth. 



Light 



light is necessary for photosynthesis, in which essential 



materials for growth are prepared, it is obvious that there can be no 



growth of the autotrophic plant in the absence of light unless supplies 

 rganic materials are available. Given such supplies 



growth will occur. Thus the root system is normally able to develop 



in darkness because it draws supplies of organic materials from the 



aerial organs, which are exposed to 

 light : while growth of both root 

 and shoot may be obtained in dark- 

 ness from such structures as seeds, 

 tubers, or rhizomes, etc., where 

 reserve substances are present, 

 provided other conditions are suit- 

 able. The growth so obtained may 

 however be of a very different type 

 from that in light, making it clear 

 that light has a direct effect on 

 growth. This statement applies 

 chiefly to the shoot, the growth 

 of roots being usually but little 

 affected by the presence of light. 

 The shoot grown in darkness is 

 distinctly abnormal in appearance, 

 and is said to be etiolated. The 

 result of etiolation varies a good 

 deal, but in many species stem- 

 elongation is much more rapid than 

 in illuminated shoots, so that the 

 etiolated shoot becomes very tall, 

 an effect which is due to a greater 

 elongation of the cells rather than 

 to more rapid cell-division. Differ- 

 entiation of the tissues is incom- 



plete in the etiolated stem, while the growth of the leaves, or more 

 ly of the leaf blades, is arrested (Fig. 91). Chlorophyll formation 



1^ inhibited, the shoot having a white or yellow colour. 

 The appearance of an etiolated shoot suggests that in addition 



to its nutritive effect, light has a direct effect on the growth of the 



«V .r'A^S- 



1 IG. 01. 



N'.nn.il fl-fti and I plants of Broad 



!'•• ID 1 .. :.i I .iba). ( • }.) 



