348 



CONDUCTING SYSTEM 



as constituents of conducting strands and of tibro-vascular bundles 

 respectively : 



Bast-fibres 

 [Extracambial Fibres] 



Composite 



Conducting. 



Strand 



Vascular 



Bundle 

 (Mestome) 



Leptome " 



Hadrome , 



Sieve-tubes and Com- 

 panion-cells Phloem 



Leptome-parenchyma 

 (including Cambiform 



cells 



Vessels (Tracheae) and N 

 Tracheides 



U. 



Parenchymatous 



Bundle-sheath 



(Conducting- 



parenchyma sheath) 



Hadrome-parenchyma 



Wood-fibres 



(Libriform cells or 



Intracambial Fibres) 



Xyleni 



Fl BRO- 



VASCU- 

 LAR 



Bundle 



Since any one of the three forms of conducting tissue that take part 

 in the formation of a composite strand may also follow a separate 

 course and carry out its functions independently of the rest, it behoves 

 us to enquire why plants in general show a distinct preference for the 

 constant association of these tissues with one another. Although this 

 question cannot be answered at all satisfactorily in the present state of 

 our knowledge, we may nevertheless indicate what general considerations 

 must regulate the further investigation of the problem. 



In the first place, it is worthy of note that water-conducting 

 elements, and particularly wood-vessels, nearly always have paren- 

 chymatous tissue (conducting parenchyma) associated with them. 

 From this circumstance it may be inferred, that there is some con- 

 nection between the functions of vessels and those of the accompanying 

 hadrome-parenchyma. It has already been suggested on a previous 

 occasion, that this connection may possibly depend upon the fact that 

 the hadrome-parenchyma takes an active part in the conduction of 

 water. But the almost constant association of parenchyma with vessels 

 is capable of a totally different explanation, if it be assumed that the 

 plastic materials utilised in the formation of the characteristic thicken- 

 ings of the vessel-walls are conveyed to the several vessels through the 

 mediation of special rows of hadrome-parenchyma cells. 



The very constant association of two tissues, namely, the protein- 

 conducting leptome and the mainly water-transporting hadrome, which 

 functionally are quite independent of one another, to form composite 

 bundles, constitutes another difficult problem. It is probable that in 

 this case a variety of circumstances have combined to produce the final 

 result. The soft and vulnerable nature of the thin-walled leptome no 



