SECT, i MORPHOLOGY 35 



unitril liy weaker cross veins. Netted or reticulately veined leaves in which tin- 

 side veins run from the median main nerve or MIDRIB are further distinguished 

 as I'iNNATKLY VEINED, or as PALMATELY 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. 



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 fre- 

 quently 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 from what it does 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 injury is afforded to the 

 Aroid (Monstera) 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, 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. 35), which 

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

 dissected and filiform in character. The pointed extremities (DRII> TIPS) of the 

 foliage leaves of many land plants, according to STAHL ( 18 ), facilitate the removal 

 of water from the leaf surface. Fleshy so-called succulent leaves, like fleshy stems, 

 serve as reservoirs for storing water. 



Heterophylly. Many plants are characterised by the develop- 

 ment 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 surround- 

 ing environment, as in the Water Crowfoot (Ranunculus aquatilis), in 

 which the floating leaves are lobed, while those entirely submerged 

 are finely divided (Fig. 35). A number of epiphytic Ferns belonging 

 to the genera Polypodium and Platycerium alternately bear stalked, 

 pinnate leaves serving for assimilation, and broad, sessile, nsually 

 cordate NEST-LEAVES which serve to collect humus ; when, as MANTLE- 

 LEAVES, they are closely appressed to the substratum they form 

 humus by their rapid decay. The humus thus collected or produced 

 is penetrated by the roots of the Fern ( 19 ). . 



The Leaf -base. In Monocotyledons the leaf-base very often 

 forms a SHEATH about the stem ; in Dicotyledons this happens much 

 less frequently. In the case of the Gramineae, the sheath is open on 

 the side of the stem opposite the leaf-blade (Fig. 36 r), while in the 



