GROWTH OF PLANTS 



133 



stems as well as retarded differentiations of tissues. Hence, 

 many authors, Palladin, for instance, are inclined to explain, in 

 part at least, changes in the form of etiolated plants by curtailed 

 transpiration in the absence of light (Art. 69). Recent observa- 

 tions, however, have shown that even short daily exposure to 

 light, 5 to 10 min., from a rather weak source, is sufficient to 

 induce etiolated plants to develop a normal form. This occurs 

 in spite of the fact that in so short a time transpiration is not 

 able to change to any extent, nor assimilation to begin, since little 



Fig. 33. — The influence of short periods of light upon the development of bean 

 leaves. Upper row: left, from plants kept in absolute darkness; center, under 

 1 min., right, 5 min. of light per day. Lower row: left, under 10 min.; center, 

 60 min. ; right, under full daylight. 



chlorophyll is formed (Fig. 33). It is probable that under the 

 influence of light there arise in the^ plant some substances of the 

 nature of hormones, which lead to changes in the character of 

 growth of leaves and stems. This assumption is confirmed by 

 the fact that violet and ultraviolet rays are of the greatest effect 

 in altering form; while the red part of the spectrum, which 

 increases assimilation and transpiration, in this case produces 

 the least effect (Fig. 34.). 



Light has also a considerable influence on the rate of growth of 

 plants. It retards growth in a definite way; and the higher its 

 intensity, the greater the retardation. Moreover, under the 



