CH. Xlll] VENATION OF LEAF 101 



equally numerous strands of stiffer, elastic, thread-like 

 supporting structures called fibres. 



The fibres contribute to the office of keeping the thin 

 and often delicate lamina or blade of the leaf i.e. the 

 flattened sheet of green tissue expanded flat in the light 

 and air, somewhat as the ribs of an umbrella keep the silk 

 stretched ; while the vessels conduct liquids, as said, from 

 one part of the leaf to another. 



In the stalk or petiole of the leaf these vessels and 

 fibres are gathered together in one, three, or five or more 

 strands and pass through the leaf-insertion into a node, 

 where they pass down into the axis of the shoot, and join 

 in the internodes with other strands coming from other 

 leaves and passing similarly down the internodes. 



Hence we say that the venation of the leaf is composed 

 of bundles of fibres and vessels shortly termed fibro- 

 vascular bundles, or even more shortly vascular bundles 

 which branch out more and more into a meshwork in the 

 lamina, and are gathered into strands, or groups of such 

 bundles, in the petiole, to again branch out in the shoot- 

 axis and join on to strands of vascular bundles in the 

 internodes. This continuity of the fibres and vessels is 

 especially important because it facilitates the passage of 

 water and other fluids from shoot-axis to leaf and vice 

 versa, and at the same time provides for the support and 

 rigidity of the leaf on the shoot-axis, a matter of peculiar 

 significance in view of the buffeting which leaves undergo 

 in a high wind. 



But the arrangement of the fibres and vessels of the 

 bundles in the shoot-axis is also by no means devoid of 

 order, as will be seen by referring to Fig. 56, which 

 represents a skeleton view of these strands in a particular 

 plant. This skeleton is obtained by removing all the 

 softer tissues which cover up the shoot shown in a, and fill 



