55 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 Tiibingen 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