RADIAL BUNDLES 353 



In roots, any departures from the typical radial structure of the 

 vascular strands are generally correlated with special environmental 

 conditions, or arise from the necessity of increasing the amount of avail- 

 able conducting tissue. Some of these modifications are connected with 

 mechanical requirements, and have accordingly been already dealt with 

 on a previous occasion (Ch. IV.). In most cases of aberrant vascular 

 structure in roots, the essential feature consists in the dilatation of the 

 central cylinder and the concomitant differentiation of a number of 

 scattered hadrome-vessels and leptome-strands on the inner side of the 

 ordinary circle of radial hadrome- and leptome-plates. This type of 



Pi3it%, 



up p c 





Fig. 142. 



T.S. through the heptarch radial bundle [stele] of an adventitious root of Primula 

 Auricula ; g, hadrome-plate ; s, leptome-strand ; p, pericycle ; it, endodermis. x 225. 

 De Bary. 



modification is illustrated by the aerial roots of many epiphytic Aroids, 

 by the ordinary roots of Musaceae and certain Palms, and by the stilt- 

 roots of Pandanus. In stout aerial roots the central cylinder is almost 

 always greatly dilated ; the consequent approximation of the water- 

 conducting elements to the periphery evidently facilitates the trans- 

 ference of water to the cortical tissues. The structural contrast 

 between the grasping and absorbing roots of some of these Aroid 

 epiphytes a contrast first pointed out by Schimper m affords a very 

 clear demonstration of the way in which this particular modification of 

 vascular structure is correlated with an increase in the demands made 

 upon the conducting system. In Monstcra deliciosa the width of the 

 central cylinder amounts to one-third of the total diameter of the organ 

 in the case of the grasping roots by which the plant is attached to its 



'A 



