128 



LECTURE VIII. 



lower down. As lor the rest, great variety prevails in the other relations 

 of the strands in the shoot-axes as well as in the leaf-stalks; and the cross 

 connections of the strands at those parts of the shoot-axes which, as so-called 

 nodes, represent swollen places close to the leaf-insertions, or occur as diaphragms 

 at the corresponding places inside hollow stems (particularly evident in the 

 Grasses), are especially to be alluded to. Besides these common (to the axes 

 and leaves) bundles, however, cauline bundles may also be found, the upper ends 

 of which, growing further in the growing-point of the shoot-axis, do not curve 

 out into leaves. In the leaf itself, especially in the lamina of green foliage 

 leaves, the vascular bundles run in the leaf-ribs described previously, where 

 they are surrounded by a special parenchymatous envelope. Their generally very 



Convallaria Inti/olia 



FIC. 131.— Course of the vascular bundles (v 



fine terminations however lose themselves, ^vithout projecting exteriorly, in the 

 green leaf parenchyma itself, and there form the fine network of veins by which 

 the broad dicotyledonous leaves especially are distinguished; or they branch 

 there in a dichotomous manner, as in many Ferns, or extend in open curves, or 

 almost straight lines in the leaves of the IMonocotyledons, — relations which we ha%'e 

 considered previously from another point of view, and shall yet treat of later on. 



The vascular bundles are generally very thin. In succulent shoot-axes, thinner 

 roots, and in the venation of the leaves, they are often scarcely so thick as a human 

 or horsehair; but occasionally they attain the thickness of a common thread, 

 or thicker. Only in the stems of the Tree-ferns, where they are broad and 

 band-like, and form a coarse net-work, 'do they reach more considerable dimensions 



