THE VEGETABLE CELL. 153 



Observ. According to the description of Dutrochet (" ^eck, sur la 

 struct intime des anim. et veget.^' 1824 ; '' Nouvell, reck, sur Vendosmose,'^ 

 1828), and Briicke {'' Miillers Archiv:' 1848, 434), the cellular tissue both 

 of the upper and under side of the articulation has a tendency to curve 

 inwards strongly. I do not find this confirmed. Of course, if a strip of 

 the vascular bundle is left connected with the inner side of the plate of 

 parenchyma above-mentioned, only the outer side of the cellular tissue 

 can expand, while expansion is prevented on the inside by the rigid 

 vascular bundle ; under these circumstances a curvature of the whole 

 plate must naturally take place. 



In uninjured articulations, the expansion of the cellular tissue 

 of the upper side maintains equilibrium with the cellular tissue 

 forming the under side, which prevents curvature of the whole. 

 But if the cellular tissue is cut away down to the central vascular 

 bundle on the upper side of the articulation of a leaf still attached 

 to the plant, the cellular tissue of the under side having now lost 

 its antagonist can pursue its expansion, ^nd the leaf thus becomes 

 at once pressed upwards at a sharp angle ; the reverse occurs 

 when the cellular tissue of the under side is removed. 



Observ. This fundamental experiment which first threw light upon 

 the anatomical system by which the movements of plants were caused, 

 was made as early as 1790 by Lindsay, but was again forgotten, so that 

 the discovery he established was made a second time by Dutrochet (" fSv/r 

 la struct, intime f^ d'c. 1824). 



According to the common statement, which rests upon the 

 experiments of Dutrochet, a leaf robbed in the above described 

 way of one side of its thickened joint, loses its power of motion 

 entirely, and after the removal of the lower portion of the cel- 

 lular tissue of its articulation can rise up no more, and can sink 

 down no more after the loss of its upper portion. But, as Briicke 

 Q'l. c." 452) correctly observed, this is not altogether true, since 

 such a leaf still performs the movements of sleeping and waking, 

 although in a much less marked degree (especially when the cel- 

 lular tissue of the under side is removed) ; and moreover, as will 

 be mentioned farther on, has not altogether lost its irritability. 



It is clear that the movements of a leaf upwards and down- 

 wards, dependant upon one-side expansion of the cellular tissue 

 of the articulation, may take place in a variety of ways. In the 

 first place, if the cellular tissue of the upper side swells up and 

 thus acquires a preponderance over that of the under side, a cur- 

 vature downwards must take place, and, vice versa, swelling of 

 the under side of the tissue must raise the leaf; on the other hand, 

 the same result must occur when the cellular tissue of one side 

 becomes lax, and thus gives that of the opposite side the oppor- 

 tunity of following its natural tendency to expand. It is possible 

 also that both these conditions may exist" at the same time. 



From the erroneously assumed immobility of a leaf, in which 



