MORPHOLOGY 



31 



leaves (Fig. 178) the veins branch oft' from one another, and gradually 

 decrease in size until they form a fine anastomosing network. In leaves 

 with parallel venation the parallel main 

 nerves are usually united by weaker cross 

 veins. Netted or reticulately veined leaves 

 in which the side veins run from the median 

 main nerve or midrib are further distin- 

 guished as PINNATELY VEINED, or as PAL- 

 MATELY veined when several equally strong 

 ribs separate at the base ■ of the leaf-blade, 

 and give rise in turn to a network of weaker 

 veins. Parallel venation is characteristic, in 

 general, of the Monocotyledons ; reticulate 

 venation, of Dicotyledons. Monocotyledons 

 have usually simple leaves, while the leaves 

 of Dicotyledons are often compound, and are 

 also more frequently provided with stalks. 

 Many plants are characterised by the de- 

 velopment of different forms of foliage leaves. 

 Such a condition is known as heterophylly. 

 Thus the earlier leaves of Eucalyptus globulus 

 are sessile and oval, while those subsequently 

 formed are stalked and sickle-shaped. In 

 other cases the heterophyllous character of 

 the leaves may represent an adaptation to the 

 surrounding environment, as in the Water 

 Crowfoot {Ranunculus aquatilis), in which the 

 floating leaves are lobed, while those entirely Fig. si. 

 submerged are finely divided. 



Part of stem and leaf o 

 s. h, Haulm ; v, leaf- 

 sheath ; fc, swelling of the leaf- 

 sheath above the node ; s, part 

 of leaf-blade ; Z, ligule. (Nat. 

 size.) 



The nerves or veins give to a leaf its necessary 

 mechanical rigidity and render possible its flattened 

 form. The branches of the veins parallel to the margin 

 of most leaves prevent their tearing : when there are no such marginal nerves in 

 large thin leaves, the lamina is easily torn into strips by the wind and rain. This 

 frequently happens to the leaves of the Banana (Musa), which, consequently, when 

 growing under natural conditions in the open air, presents quite a different appear- 

 ance than when grown under glass. The leaves of the Banana, after becoming 

 thus divided, offer less resistance to the wind. In a similar manner the leaves of 

 Palms, although undivided in their bud state, become torn even during the process 

 of their unfolding. A similar protection from inj ury is afforded to the Aroid (Mon- 

 stera) by the holes with which its large leaf-blades become perforated. Equally 

 advantageous results are secured by many plants whose leaves are, from their very 

 inception, divided or dissected. The submerged leaves of aquatic plants, on the 

 other hand, are generally finely divided or dissected, not only for mechanical 

 purposes, but also to afford a more complete exposure of the leaf surface to the 

 water. Accordingly, in such water-plants as Ranunculus aquatilis (Fig. 197), 

 which possess both floating and submerged leaves, it is generally the latter only 



