550 History of the Doctrine of [BOOK in. 



movements of the parts of plants are produced by the spiral 

 vessels, or, which meant the same thing in those days, by the 

 vascular bundles. It was an important event therefore when 

 Dutrochet proved in 1822, that the movements of the leaves 

 of Mimosa were due to the alternate expansion of the antago- 

 nistic masses of parenchyma in the pulvinus or cushion of suc- 

 culent tissue found at the articulation, and that the central 

 vascular bundle follows passively their curvatures. Lindsay 

 had indeed arrived at the same conclusion from similar experi- 

 ments as early as 1790, but his unprinted essay on the subject 

 was first produced by Burnett and Mayo in 1827. Meanwhile 

 Dutrochet had also found that light influences the movements 

 of the leaves in different ways ; alternation of light and dark- 

 ness excites them to motion, while leaves which have become 

 rigid in continued darkness are restored by light to their normal 

 condition of sensitiveness. 



Much attention was bestowed in the period between 1820 

 and 1830 on various questions connected with the movements 

 of the organs of plants. In 1826 the faculty of medicine in 

 Tubingen offered a prize for an essay on the peculiar nature 

 of tendrils and climbing plants, which was intended to bring 

 into discussion all the points which required to be cleared up 

 before a more thorough understanding of the whole subject 

 could be obtained. The two essays which gained the prize 

 were published in 1827. One was by Palm, the other by von 

 Mohl, both of very different value. Palm's essay is a good 

 and careful college-exercise ; but there is nothing of this char- 

 acter in von Mohl's. The skill of the composition, the exact 

 knowledge of the literature of the subject, the wealth of per- 

 sonal experience, the searching criticism, the prominence given 

 to all that is fundamental and important, the feeling of cer- 

 tainty and superiority which the book inspires, all unite to 

 make the reader forget that it is not the work of a mature and 

 professed naturalist, but of a student of two-and-twenty years of 

 age. This academical prize-essay on the structure and twining 



