THE AXGIOSPERMAE : LEAVES 95- 



to come directly from the ground are classed as radical leaves. They are 

 specially frequent among Monocotyledons, but are also characteristic of 

 dicotyledonous biennial plants, in which the aerial stems only emerge from 

 the ground level and elongate during the second year of life. 



Venation of Leaves. 



The term " vein " is often used as if it were synonymous with a vascular 

 bundle. This is only true of the smaller veins, for the midrib and the larger 

 veins ot the leaf may contain a group of vascular bundles and sometimes 

 indeed have a structure which closely resembles that of a stem. ^Moreover, 

 when the leaves of certain plants, such as Beoonia or Torenia are emploved 

 as cuttings, and inserted in the soil until they develop adventitious buds and 

 roots, the growth of these new organs may stimulate development in the 

 larger leaf veins, leading to the formation of quite considerable amounts of 

 secondary wood, arranged in a circular zone, exactly as in a stem. 



These anatomical resemblances have been used as arguments in favour of 

 the axial nature of the leaf, a question, however, which we have already 

 sufficiently discussed (see p. 947). 



The larger veins are usually encased in a sheath of parenchymatous or 

 sclerenchymatous cells which cuts them off from the mesophvll. These 

 sheaths are generally more developed on the lower than on the upper side 

 and consequently they stand out as more or less prominent ribs on the lower 

 leaf surface, though less commonly they may also protrude above the level on 

 the upper surface. The smaller veins, on the contrary, are in close contact 

 with the mesophyll tissue. 



Veins serve both mechanical and conducting functions in the leaf, as in 

 the stem. In many leaves, especially in Monocotyledons with very long 

 leaves, the sclerenchymatous sheaths of the veins are extremely strong and 

 they are thickest above and below the vascular bundle of the vein, presenting, 

 with the latter between them, a girder section which is xtvy resistant to bending 

 strains. 



It is generally said that the venation of the leaves in Monocotyledons and 

 Dicotyledons is quite distinct. The monocotyledonous leaf is usually broad- 

 based and clasps the stem at the node. It receives a considerable number of 

 leaf trace bundles from the stem, and these form veins which follow a more 

 or less straight and parallel course towards the apex of the leaf (Fig. 943), 

 though in leaves which are broad in the middle part, the veins may be 

 markedly bowed outwards and form a series of parallel arcs. The dicotyle- 

 donous leaf has usually a very limited number of trace bundles, which branch 

 repeatedly in the lamina and anastomose to form a network. The variety 

 of patterns is very great and seems to be determined more by factors of 

 heredity and relationship than by immediate physiological needs. 



In Monocotyledons there are relatively few cross-connections between 

 the main veins, and such as there are follow a straight transverse course 

 from vein to vein (Fig. 944). The network in the Dicotyledon, however, 



