SECT. I MORPHOLOGY 35 



VEINED or NETTED VEINED. Ill parallel venation the veins or nerves run either 

 approximately parallel with each other or in curves, converging at the base and 

 apex of the leaf (Fig. 35 s) ; in netted veined leaves (Fig. 190) the veins branch off 

 from one another, and gradually decrease in size until they form a fine anastomos- 

 ing 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 distinguished 

 as PINNATELY 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 tlie 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. Their distribution is usually such as to allow of con- 

 duction taking place by the shortest possible routes throughout the leaf-blade (^'). 

 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 appearance 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 pro- 

 cess of their unfolding. A similar protection from injury is afforded to the Aroid, 

 Monstera. hy the holes with which its large leaf- blades become jierforated. 

 Equally advantageous results are secured by many plants whose leaves are, from 

 their very inception, divided or dissected. The pointed extremities (duip tips) of 

 tire foliage leaves of many land plants, according to Stahl (^^), facilitate the 

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

 fleshy stems, serve as reservoirs for storing water. 



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. 35 v), while in the Cyperaceae it is com- 

 pletely closed. The sheath of the Grasses is prolonged at the base of 

 the lamina into a scaly outgrowth, the ligule (/). Such a sheath, while 

 protecting the lower part of the internodes, which remain soft and in 

 a state of growth, gives them at the same time rigidity. Stipules 

 are lateral appendages sometimes found at the base of leaves. When 

 present they may be either small and inconsijicuous (Fig. 36 nb), or 

 may attain a considerable size. When their function is merely to 

 protect the young parts in the bud, they are usually of a brown or 

 yellow colour, and are not persistent ; whereas, if destined to become 

 assimilatory organs, and to assist in providing nourishment, they are 

 green, and may assume the structure and form of the leaf-blade, which 

 sometimes becomes modified and adapted to other purposes (Figs. 48, 

 49). Normally, the stipules are two in number, that is, one on each 



