196 XV. THE LEAF VASCULAR BUNDLE ENDINGS. 



(&'), are interposed in the spongy parenchyma. The chief veins 

 and the lateral veins of the first order, project strongly from the 

 under surface of the leaf in the form of ribs. This projecting 

 part is about as thick again as the other parts of the leaf. The 

 vascular bundle traverses the projecting rib. This latter is 

 covered with elongated epidermal cells, to which follow elongated 

 collenchymatous cells. To these adjoin cells, each of which 

 contains a simple crystal ; and then follows a multilamellar 

 sheath of sclerenchyrna fibres, which envelops the whole bundle. 

 On the upper side, the palisade layer is interrupted in a narrow 

 strip over the vascular bundle, and is replaced by collenchyma, 

 above which is a narrow strip of elongated epidermal cells (cf. 

 Fig. at ep'"). A layer of chlorophyll-containing cells surrounds 

 the sclerenchyma sheath of the vascular bundle, and the cells of 

 the spongy parenchyma join on to these. 



Mechanics of the Leaf . The ribs represent the mechanical 

 system of the leaves, which must be made firm against flexure. 

 The ribs may again be, as far as their mechanical development is 

 concerned, likened to girders. The girders are arranged sym- 

 metrically with regard to the surface of the leaf, the plane of the 

 girder being perpendicular to this surface. The upper side of the 

 leaf is " stayed " especially against traction, the under side against 

 compression. The girders in this case are arranged, in each rib, 

 in the form of an I, the vascular bundle forming the "filling". 

 The mechanical capacity of the under part of the girder, con- 

 structed against compression, is heightened by its removal as 

 deeply as possible out beyond the under surface of the leaf into 

 the projecting rib. By means of the veins the leaf-blade is tightly 

 expanded, and attains thereby the firmness necessary to protect 

 it from tearing. 



Smaller vascular bundles, like that of the figure (77), are only 

 protected on the upper and under side by a few sclerenchyma 

 fibres. The ultimate branchlets of the veins are devoid of scleren- 

 chymatous cover, and are invested directly by the sheath of 

 parenchyma. The smaller vascular bundles are accompanied 

 on wood and bast sides by crystallogenous cells (fc). Above 

 and below them the epidermal cells are somewhat elongated, 

 and form shallow, depressed streaks. From the epidermal cells 

 upon the veins arise long hairs, like sclerenchyma fibres, which 

 are, however, for the most part exfoliated in fully-developed 

 leaves. 



