282 PHYSIOLOGICAL DEVELOPMENT. 



frameworks, are also in any way determined by the currents 

 of sap carried? There are some such grounds. 



As just indicated, the only places where we may look 

 for evidence with any rational hope of finding it, are 

 places where some local requirement for vessels has arisen 

 in consequence of some local development which the type 

 does not involve. In these cases we find such evidence. 

 Good illustrations occur in those genera of the Cactacece, 

 which simulate leaves, like Epiphyllum and Pliyllocactus. 

 A branch of one of these is outlined in Fig. 256. As before 

 explained this is a flattened axis; and the notches along 

 its edges are the seats of the axillary buds. Most of these 

 axillary buds are arrested; but occasionally one of them 

 grows. Now if, taking an E pi pkyllum -shoot which bears a 

 lateral shoot, we compare the parts of it that are near the 

 aborted axillary buds with the part that is near the de- 

 veloped axillary bud, we find a conspicuous difference. In 

 the neighbourhood of an aborted axillary bud there is no 

 external sign of any internal differentiation; and on hold- 

 ing up the branch against the light, the uniform trans- 

 lucency shows that there is no greater amount of dense 

 tissue near it than in other parts of the succulent mass. 

 But where an axillary bud has developed, a prominent rounded 

 ridge joins the midrib of the lateral branch with the midrib 

 of the parent branch. In the midst of this rounded ridge 

 an opaque core may be seen. And on cutting through it, this 

 opaque core proves to be full of vascular bundles imbedded 

 in woody deposits. Clearly, these clusters of vessels imply 

 transformations of the tissues, caused by the passage of in- 

 creased currents of sap. The vessels were not there when 

 the axillary bud was formed; they would not have de- 

 veloped had the axillary bud proved abortive; but they 

 arise as fast as growth of the axillary bud draws the sap 

 along the lines in which they lie. Verification is obtained 

 by examining the internal structures. If longitudinal 

 sections be made through a growing bud of Opuntia or 



