408 PHYSIOLOGY OF GROWTH. 



direct sunlight, e.g. at a window. The part of the plant under the 

 box is prevented from becoming 1 too warm by means of suitably 

 placed screens. In my experiments I found that the portions of 

 bean stems growing in the dark were able to twine round a 

 support. 



1 See Sachs, Botan, Zeitung, 1863, Beilage. 



2 See Detmer, Versuchsstationen, Bel. 16, 



8 See also Sachs, Arbeiten d, botan. Inst.in Wurzbarg, Bd. 3, p. 372. 



165. The Causes of Etiolation. 



It is a fact that many kinds of plants, when grown in absolute 

 darkness, produce abnormally long stems and small leaves. The 

 first question which presents itself is whether the peculiar forma- 

 tion of etiolated plants is not perhaps due to the arrest, in darkness, 

 of the assimilatory activity of their leaves. To obtain an answer 

 to this question we conduct the following experiments with seed- 

 lings of Raphanus sativus. 1 



Raphanus seeds are soaked in water, and then laid on coarse 

 sand contained in two small flower-pots, and watered with dilute 

 food solution. Each flower pot is then placed in an arrangement 

 such as was described in 16, and represented in Fig. 18. One 

 apparatus is exposed to bright diffused daylight, the other is placed 

 close to it, under a cardboard box covered with black paper. The 

 seeds soon germinate, and while the shaded seedlings produce long 

 hypocotyls and cotyledons of small width and length, the illu- 

 minated seedlings, which have become green in colour, present a 

 perfectly normal appearance. These last however, although they 

 possess chlorophyll, cannot assimilate, for they are surrounded by 

 air devoid of Carbon dioxide, and therefore failure of assimilatory 

 activity cannot be regarded as the cause of the peculiar formation 

 of etiolated plants.* 



To obtain a closer insight into the causes of etiolation, it is im- 

 portant to make the following experiment. A number of Raphanus 

 seeds are selected as nearly as possible of the same size. We 

 determine the weight of each individual seed, and only use for the 

 experiment such as are approximately of the same weight. After 



* But light may be of importance for the formation of specific substances 

 wbich are necessary for normal development of the leaves. These bodies may 

 provisionally be designated leaf-forming substances. 



