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the leaves, whether the nerves of the leaves remained during 

 the process active or passive, &c., and the following results 

 were arrived at : that leaves with simple nerves and without 

 petioles change their position from the unnatural to the natural 

 as well as those with petioles ; and 2ndly, that the parenchyma 

 is the cause and not the nerves. Further observations re- 

 specting the mechanism producing the motion of leaves with 

 and without petioles gave the following results : 1. All leaves 

 with simple veins have the power of self inversion ; 2. The 

 apparently unpetiofated leaves in which the veins are diffused 

 in a different manner, move by a bend in their point of attach- 

 ment; 3. The short and stiff as well as the long and slender 

 petioles are unfavourable to the motion ; 4. When the petiole 

 is not too stiff or long, the inversion of the leaves is produced 

 by a semi-inversion lengthwise and also by a bend of the pe- 

 tiole ; 5. In folia pelt at a the motion takes place partly by a 

 bend of the petiole itself, partly by a change in the direction 

 of the leaf with reference to the petiole. 



M. Dassen then proceeds to the examination of the causes 

 which produce the motion of leaves : various plants stationed 

 in pots were left to grow turned from the light, and some even 

 without light in closed boxes. The result was highly remark- 

 able : the leaves of those plants w r hich could not turn them- 

 selves round died, but the remainder were inverted quite as 

 quickly in the dark as in the light, whence M. Dassen arrives 

 at the conclusion, that light was no more the cause of the di- 

 rection of the leaves upwards than darkness is the cause of the 

 downward direction of the root. Neither can the action of 

 heat or that of moisture be regarded as the cause of this mo- 

 tion. Finally M. Dassen passes in review those motions of 

 the leaves which take place constantly in the course of one day, 

 and even without swellings ; these are the phaenomena which, 

 as is well known, were regarded by Linnaeus as the Sleep of 

 Plants. M. Dassen considers Linnaeus's explanation as an 

 error into which that great man fell, as well as all those who 

 have merely copied almost word for word from him respecting 

 this point. The memoir On the Sleep of Plants, by E. Meyer, 

 which I noticed in my report for 1835, as highly interesting 

 and full of laborious research, is especially attacked. I cannot 

 however agree with M. Dassen : for all the valuable observa- 



