152 M. Marc Micheli on some Recent 



older tissues is of no use to it. In darkness no starch is pro- 

 duced ; aud it is therefore not surprising that development is 

 arrested. This view is so accurate that certain cotyledons 

 destined to display a foliaceous structure stop growing in 

 darkness at the moment when they ought to issue from the 

 ground, although their cells are still full of the sugar or oil 

 which was accumulated in the seed. 



The exaggerated length of the internodes is due to very dif- 

 ferent causes, and is related to the phenomena of tension 

 which always intervene in stems between the medulla, or ac- 

 tive part, on the one hand, and the ligneous and cortical cells, 

 or passive parts, on the other. 



From an anatomical point of view the etiolated internodes 

 are distinguished by presenting all the characters of very young 

 internodes just issuing from the bud ; the thickening of the 

 walls of the ligneous and cortical cells which characterizes 

 adult stems is here completely absent. This thickening, in- 

 deed, is related, by bonds which are not yet very exactly un- 

 derstood, to the presence of leaves on the internode. In 

 darkness, the leaves not being developed, the cells retain the 

 primitive thinness of their membranes. 



This being understood, the elongation of the etiolated stems 

 is easily explained, thanks to the intervention of two factors. 

 In normal stems 4;he medulla has always a tendency to elon- 

 gate ; it is the peripheral layers that arrest it ; in young stems 

 these are subjected to a tension strong enough to cause them 

 to shorten considerably when they are isolated. But in pro- 

 portion as their walls become thickened the resistance becomes 

 more effective, and we see this in the fact that their contraction 

 when they are separated from the rest becomes less and less. In 

 darkness their walls do not thicken, and nothing is opposed to 

 the elongation of the medullary cells. This is the first factor. 



With regard to the pith itself, M. Krauss has already 

 shown, in a former work*, that it has the property of elongation 

 solely by the interposition of aqueous molecules between the 

 cellulose molecules. This interposition may take place in 

 the etiolated as in the normal plant ; the pith is therefore the 

 only part of the plant which continues to grow actively in the 

 dark. This growth is precisely the second factor of the elon- 

 gation of the internodes ; and by combining it with the absence 

 of resistance in the peripheral layers, we can understand that 

 considerable results may be produced. 



By the side of the effects of light, the investigation of those 

 of temperature quite naturally finds its place. 



* Botan. Zeit. 1867, Nos. 17, 18. 



