384 



CONDUCTING SYSTEM 



it were, carry the conducting strands with them. The spiral torsion 

 of the bundles also seems to be a device for increasing the mechanical 

 strength of the stem (cf. Ch. XIV.). 



Numerous modifications of the Palm-type are known, but only a 

 few of them can be mentioned here. In ordinary, hollow-stemmed 

 Grasses, the true form of the vascular system is obscured owing to the 

 great elongation of the internodes. Here the haulms appear to be 



traversed by parallel bundles, which descend 

 perpendicularly and are connected at the nodes 

 by an abundantly branched network of trans- 

 verse anastomoses. The development of such 

 a nodal vascular network is partly correlated 

 with the production of axillary shoots, but 

 also, doubtless, serves to ensure the access of 

 a sufficiency of plastic material to the overlying 

 intercalary meristematic zone. In addition the 

 nodal anastomoses have a mechanical signi- 

 ficance, inasmuch as they act as cross-ties 

 between the main girders. 



In the haulms of many Cyperaceae and 

 Juncaginaceae the longitudinal bundles are 

 likewise linked together by means of numerous 

 transverse connections ; these are embedded 

 in the parenchymatous diaphragms that divide 

 the large air passages into a series of compart- 

 ments. The mechanical function of such 

 " mestome-anastomoses " has already been dis- 

 cussed (Ch. IV.) ; their anatomical composition, 

 however, shows that they are also concerned 

 with the conduction of water ; for they consist 

 of narrow annular or spiral (rarely pitted) 

 tracheides, together with more or less elongated, 

 and in part thickwalled, parenchymatous elements. In all probability 

 it is the photosynthetic activity of the haulms in the Cyperaceae 

 and Juncaginaceae, that accounts for the presence of numerous 

 small cross-connections between the longitudinal water-channels. The 

 parenchyma which occupies the intervals between the peripheral 

 girders, contains abundance of chlorophyll, and is indeed often 

 developed as typical palisade-tissue, while in most species of Cypcrus 

 the vascular bundles are surrounded by the green sheaths which 

 have already been referred to in a different connection (p. 284). 

 As the transpiratory activity of such haulms must be relatively 

 considerable, it is not surprising to find arrangements, which enable 



Fir;. 153. 



Simplified diagram of the 

 course of the vascular bundles 

 in stems of the "Palm-type." 

 6, leaf-base ; v, growing-point. 

 After Falkenberg (from Sach's 

 Lectures). 



