VASCULAR SYSTEM IN LEAVES 



387 



freely without undergoing fusion. This is the simpler, more un- 

 specialised type, and mostly occurs in leaves which never require a 

 very large supply of water and nutrient materials, either on account of 

 their small size, or because their transpiratory activity is slight and their 

 photosynthetic capacity (in the case of foliage-leaves) low ; it is exempli- 

 fied by the leaves of Mosses, by the small scale-like foliar organs of 

 Equisctum, Casuarina, and Ephedra, by the foliage leaves of all Conifers 

 and of many Ferns, by the small Eataphylls and floral leaves of many 

 Augiosperms and, finally, by the submerged leaves of certain water- 

 plants (Batrachium, Myriophyllum, the 

 Hydrilleae). In all these cases, the leaf 

 is traversed, either by a single medium 

 strand, or by a system of bundles which 

 branch but do not anastomose. 



The second principal scheme of vena- 

 tion is characterised by the presence of 

 numerous anastomoses, which link together 

 the different branches of the vascular 

 system. In the majority of Monocoty- 

 ledons, the main strands all extend in 

 straight lines or along slightly curved 

 paths to the tip of the leaf (Fig. 154); 

 here the anastomoses take the form of 

 slender cross-connections between the prin- 

 cipal veins. In almost all Dicotyledons, 

 on the other hand, the bundles put forth 

 branches in all directions in the plane of 

 the leaf, which anastomose very frequently, 

 the whole system assuming the character 

 of a dense network, with its ultimate rami- 

 fications [bundle-ends] terminating blindly 

 in its meshes (Fig. 155). According to Schuster, the disposition of 

 the smaller branches is such that the meshes form " areas of minimum 

 perimeter." This arrangement enables every part of the leaf to obtain 

 its proper share of water and mineral salts with the smallest possible 

 expenditure of vascular tissue, and, of course, similarly facilitates the 

 removal of synthetic products from the mesophyll. 



Generally speaking, the vascular network is most extensive and 

 most closely woven in leaves which transpire very actively. The 

 great difference in transpiratory activity between petals and foliage- 

 leaves of the same plant is clearly reflected in the development 

 of the vascular system in the two sets of organs (Fig. 156). A 

 similar contrast prevails, with regard to the development of the foliar 



Fig. 155. 



Venation of the leaf of Salve grandi- 

 folia,. After Von Ettingshausen (from 

 Sachs, Lectures). 



