386 



CONDUCTING SYSTEM 



Isolated leptome-bundles are found in Ghamaedorea, in some of the roots 

 of Musa rosacea, and probably also in the genera Monstcra and Raphido- 

 phora. The accessory hadrome-strands anastomose more or less freely in 

 the Cyclanthaceae, and also in Chamacrops hvmilis and Areea rubra. 

 It is unusual for accessory leptome-strands to be connected by anastomoses ; 

 more often the outermost strands unite with the peripheral leptome- 

 plates, or adjacent accessory strands may fuse in pairs. It appears, 

 therefore, that conducting strands which occur as accessory structures 



within the primary central cylinder of a 

 root, generally serve in the first instance 

 for the transportation of materials through 

 the whole length of the organ, or at any 

 rate over considerable distances. 



Where lateral roots are modified to form 

 tuberous storage organs (Dioscorea Batatas, 

 Sedum Telcphiwuri), the typical axile strand 

 is replaced by, or resolved into, a number 

 of distinct collateral bundles ; it can hardly 

 be doubted that this exceptional arrangement 

 tends to facilitate the deposition and removal 

 of reserve-materials. The root-tubers of the 

 Ophrydeae are characterised by the presence 

 of several radial bundles, each enclosed within 

 a typical endodermis. Van Tieghem believes 

 that each of these "bundles" really represents 

 an independent central cylinder, and that the 

 tuber owes its origin to the (phylogenetic) 

 fusion of several distinct lateral roots. 



Fir,. 154. 



C. COURSE of the: vascular bundles 



IN LEAVES. 



Venation of the leaf of Convallaria 

 latifolia. After Von Ettingsbausen 

 (from Sachs, Lectures). 



The arrangement of vascular bundles in 

 petioles presents no features of special 

 interest ; attention may therefore forthwith be directed to flat leaf-blades 

 and to the flattened axial organs which resemble leaves. 



It is a familiar fact, that leaves differ greatly among one another 

 in respect of what is generally termed their " venation " ; this character 

 has indeed been widely employed for taxonomic purposes, especially by 

 palaeontologists, and a special terminology of venation has been invented 

 to this end. The study of leaf-venation from the physiological point of 

 view, on the other hand, is a comparatively recent development. 



All the various forms of venation may be referred to one or other 

 of two principal types. In one case the bundles are isolated, and end 



