676 JOUKNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



dotted), there are radial stiffening bands connecting this with the circum- 

 ference, acting as webs to the combined girders making the inner and 

 outer cylinders. The centre is hollow. If the inner circle be reduced in 

 size and the radial bars elongated, it will be at once perceived that we 

 have a construction of any ordinary wheel with spokes. There is no 

 need for great strength in the latter, as the combined girders bear all the 

 stress, so that they are often made extremely slender, as in the wheels of 

 a bicycle. 



Climbing Stems. — It is well known that the structure of the stems 

 of woody climbing plants is very anomalous : i.e. as compared with an 

 ordinary, erect stem of a shrub or tree. The explanation is that the 

 abnormal distribution of wood, pith, medullary rays, and bast, as seen in 

 woody lianas of tropical forests, is the result of a response to the strains 

 to which such plants are inevitably subjected. Though there is great 

 variety among them, the internal features are generally to be seen in 

 feeble wood, much superficial and irregularly formed corky tissue, 

 together with large and many vessels. Such a combination allows of 

 much elasticity and resiliency. The external forms of such stems are 

 also various. Thus, some develop long ridges, and, becoming twisted. 



Fig. 288.— Tendril of Bryony (Bryonia dioica,. (After Darwin.) 



they closely resemble a cable in appearance, strength being acquired in 

 both alike by the spiral twisting. 



Others are flattened like broad woody ribbons, bulging in and out like 

 waves, strengthened by flange-like ribs at the edges. Such is the case 

 with Caulotrctus, the Monkey Ladder, and Bauliinia. But whatever the 

 form may be, it is specially adapted for strength and to sustain any strains. 



In tendrils, as of Bryony and Passion-flowers, as soon as they have 

 caught anything they coil up into spirals, but the number of the " turns " 

 is equally distributed both ways. This enables the accumulated force 

 of one set of coils to be neutralised by that of the other, while the whole 

 series allows of considerable amount of play, which prevents the tendril 

 being torn in stormy weather. Fig. 288, taken from Darwin's work on 

 " Climbing Plants," illustrates this coiling in the tendril of the Bryony. 



Mechanical Movements in Leaves. — The use of mechanical con- 

 trivances in these organs is very frequent, if not universal. Thus, in his 

 book on "Insectivorous Plants," Darwin has shown how the longer 

 circumferential " tentacles " of the Sun-dew will curve over any fragment 

 of nitrogenous matter placed on the shorter ones in the middle of the 

 blade, and how the two halves of the blade of the Venus' Fly-trap will 



