LECTURE XVI. 



that the average length of the hypocotyl and primary root 

 taken together was about the same whether the plant had 

 grown in light or in darkness, a result which was confirmed 

 with regard to other plants by Lasareff, the hypocotyl being 

 longer and the root shorter in the etiolated than in the normal 

 plant. Again, C. Kraus accounts for the usual smallness of 

 the leaves of etiolated shoots by regarding it as a consequence 

 of the excessive elongation of the internodes. As a matter of 

 fact, this is not the case, as Godlewski has shewn. But 

 assuming it for a moment, we must admit that the converse 

 also will be true, that active growth of the leaves will restrain 

 the excessive growth of the internodes in darkness. Godlewski 

 has found, however, that the excessive growth of the hypocotyl 

 of the Radish in darkness is only very slightly diminished 

 when the cotyledons are enabled to grow by exposure to 

 light. Naturally when there is only a limited supply of 

 plastic material the growth of any one organ will eventually 

 be affected by the demands made upon the store by the other 

 growing organs. But this does not at all touch the real point 

 at issue, that, namely, whilst there is an adequate supply 

 of plastic material, the growth of some organs should be 

 promoted by the absence of light, and that of others hindered. 

 Another explanation of the excessive elongation of etiolated 

 internodes is that offered by Rauwenhoff, who concludes that 

 it is due to the influence to gravity which, as we shall learn 

 more fully hereafter, tends to cause stems to grow erect. It 

 is difficult to reconcile this view with the fact that an excessive 

 elongation of the internodes in darkness takes place when the 

 plant is hung upside down, so that its normal relation to 

 gravity is reversed, and also when the growing plant is made 

 to rotate slowly about a horizontal axis in darkness so that 

 the effect of gravity is eliminated. 



The only satisfactory explanation that can be given is that 

 when a shoot grows in the light, the light exercises a tonic 

 influence upon the growing cells such that their growth is 

 retarded, whereas in darkness, in the absence of this tonic 

 influence, their growth is more rapid. We will postpone a 

 fuller consideration of this matter for a short time. 



