THE VEGETABLE CELL. 157 



The external action of ponderaTble or imponderable agents is 

 necessary to the production of all the movements hitherto men- 

 tioned ; but besides these, there occur in isolated cases motions 

 which, so far as existing experience reaches, are wholly independ- 

 ent of external influences. 



It is held as an enigmatical phenomenon that a twining plant 

 which stands at a distance of one or more feet from its support, 

 reaches this in order to grow up it ; the cause of it is sought 

 sometimes in a mysterious power in these plants of seeking out 

 the supports, sometimes in their asserted property of turning 

 away from light, &c., but the matter is explained in the simplest 

 manner by a peculiar raotion with which the stems of these plants 

 are endowed. The younger internodes of twining stems are quite 

 straight and their vascular bundles run, like those of other stems, 

 parallel with the axis of the stem ; but when an internode has 

 attained a certain age it begins to curl (according to the species 

 of plant, to the right or left) around its own axis, in consequence 

 of which the vascular bundles acquire a spiral course. This curl- 

 ing occurs only in a short length of the stem in each single period 

 of time, but it advances gradually from below upwards, proceed- 

 ing from one part of the stem to another, without ever becoming 

 recurrent The upper part of these stems, always slender and 

 pliable, hangs over in a curve ; and since it must follow the curl- 

 ing of the lower part, it is continually carried round in a circle 

 like the hand of a clock ; then if a solid body stands within the 

 circle described by the point of the stem, the latter becomes pressed 

 upon the solid body, the irritability peculiar to it becomes excited, 

 and thus the twining round the support is produced. (For the 

 more minute details of this process, see my essay above referred 

 to.) We do not at present possess an explanation of these move- 

 ments and of the circumstance that they occur constantly, either 

 to the right or to the left, in each species of plant i but it cannot 

 be doubted that the movement here also has its origin in the 

 parenchymatous cellular tissue, smce the perfectly visible distinc- 

 tion between the stems of twining plants and those of other vege- 

 tables, depends on the relative abundance of succulent cellular 

 tissue in the former, and since in many plants, e. ^., in Cynanche 

 VincetoxicuTyh, the stem becomes more inclined to twine, the 

 more its succulence is favoured by the shade and moisture of the 

 locality in which it grows. 



Ohserv. TBe circular movement of the stem just described has nothing 

 to do with the twining round the support ; in fact, that part of the stem 

 which has undergone the torsion is incapable of twiniag round a support, 

 and the moyement of curvature, which causes the twining, occurs only in 

 the yotmger part of the stem, the fibres of which still retain their straight 

 course This may be caused not only by the younger parts of the st^n 

 being the softer, more juicy, and, in consequence of this, more moveable, 

 but alfao, and priacipally, by the circumstance that in old curled parts of 



