660 CONSERVATIVE ORGANS. [LKCT. XI 



cumstances favouring their development occur: 

 and the stem having ceased to throw out claws 

 on the side next the wall where they origin- 

 ally appeared, a smaller degree of shade and 

 moisture is now sufficient for the development of 

 those which remain unprotruded. In plants of 

 Bignonia radicans, which are not trained upon a 

 wall, but supported by sticks in pots in the 

 greenhouse, the claws are put forth in clusters at 

 four opposite points around the stem near the base 

 of each pair of leaves; for here, circumstances being 

 equal on all sides, the power of the stem to pro- 

 trude ciaspers is not diminished in one part by 

 being increased on another, and, consequently, 

 the natural effort takes place on all sides, in the 

 same degree. 



In structure, the radicular claw closely re- 

 sembles both the petiole and the radicle. In the 

 Ivy it consists of a cortex inclosing a cellular 

 mass in which the vessels are embedded, and ar- 

 ranged in five distinct fasciculi, which evidently 

 proceed from those of the ligneous, or rather al- 

 burnous part of the stem, although Mr. Knight 

 has remarked (Phil. Trans. 1812) that the claw of 

 the Ivy " appears to be a cortical production only. v 

 The vascular fasciculi are given off nearly at 

 right angles with those of the stem; and are 

 placed at equal distances from one another, in a 

 circle nearer to the centre than to the cortex of 



