GROWTH 303 



etc., while very often no sporangium (Mucoraceae) or pileus 

 (Basidiomycetes) is formed, or else are underdeveloped. Elonga- 

 tion in darkness is likewise pronounced with sprouts of the potato 

 which are abundantly supplied with nutrient substances from the 

 tuber. In spite of numerous attempts to explain the causes of 

 the peculiar alterations in form of etiolated plants, our knowledge 

 of the subject remains very scanty. Growth in an atmosphere 

 saturated with water vapor also leads to poor development of 

 leaves and elongation of stems as well as retarded differentiation 

 of tissues. Hence, many authors, Palladin, for instance, are 

 inclined to explain, in part at least, changes in the form of etio- 

 lated plants by curtailed transpiration in the absence of light 

 (Art. 51). Recent observations, however, have shown that even 

 the shortest daily exposure to light, 5 to 10 min., proceeding, more- 

 over, 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 no chlorophyll is formed. 

 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 ultra-violet 

 rays are of the greatest effect in changing form, while the red 

 part of the spectrum, which increases assimilation and transpira- 

 tion to the highest degree, in this case produces the least effect 

 (Fig. 121). 



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 influ- 

 ence of light, plants pass more rapidly through the grand period 

 of growth, the period of elongation terminating very early; before 

 the growing organs have had time to elongate. Consequently, 

 shoots that have developed in strong light are always shorter than 

 those grown in shade. 



The behavior of the seedlings at the moment they have pushed 

 their way through the earth and emerge into light, is very inter- 

 esting: their growth is instantly checked. This delay affects 

 equally all parts remaining in the soil. Under the influence of the 

 light, there evidently arises in the tip some growth-delaying stimu- 

 lus which is transmitted to the underground parts. Some authors 



