96 BOTANY PART i 



inner portion, between the xylera of adjacent bundles, is formed of sclerenchyma. 

 This contrasts with the outer parenchymatous portions situated in the region of 



the phloem. 



Subterranean shoots and submerged plants which have to withstand pulling 

 forces have their mechanical tissue more or less centrally placed ; it may be in 

 the pith. 



Course of the Vascular Bundles. In accordance with their 

 functions the vascular bundles form continuous strands which in macer- 

 ated preparations may be followed from the root-tips to the extremity 

 of the leaves. This can be done by letting herbaceous plants lie in 

 water until all the tissues except the more resistant vascular bundles 

 have decayed and disappeared. 



The bundle of the root is traced to the base of the shoot, where it 

 is continuous with the more complicated system of vascular bundles 

 (cf. p. 137). The bundles in the stem may be traceable to the apex 

 without passing into the leaves. Such bundles are termed CAULINE, 

 and contrast with purely FOLIAR bundles which immediately on 

 entering from a leaf unite with cauline bundles. 



Thus in the Pteridophyta there may be a network of cauline bundles or a 

 single central bundle (Lycopodium, etc.) with which the foliar bundles unite on 

 entering from a leaf-base. 



As a rule, however, the bundles of the shoot bend outwards into 

 leaves and are COMMON bundles, the upper portion of which belongs 

 to a leaf and the lower portion to the stem. One or several such 

 bundles pass into a leaf and form collectively what is known as the 

 leaf-trace. The vascular system of the stem in the seed plants consists 

 as a rule entirely of these leaf-traces or common bundles. 



The stems of some Dicotyledons (Begonia, Aralict) possess cauline bundles in 

 the pith enclosed by the circle of common bundles. At the nodes these cauline 

 bundles, which may be arranged in a ring concentric with the common bundles, 

 are connected with the latter. 



The leaf-trace bundles may remain separate from one another in 

 the stem, but usually each descending bundle of the trace ends by 

 joining another bundle that has entered from a lower leaf. A 

 splitting or forking of the bundle may precede this junction. Such a 

 reticulate arrangement of the bundles ensures a uniform distribution 

 of the water supply, since each bundle of the stem as a consequence of 

 its subdivision provides water to a larger region of the shoot. The 

 general course of the bundles differs in different species according to 

 the length of the free course of the single bundles of the trace, the 

 course they follow, and the subdivision they undergo. The arrange- 

 ment of the leaves naturally determines the places of entry of the 

 leaf-traces into the stem. Their course in the stem is, however, quite 

 independent of the leaf arrangement, and can be very different for one 

 and the same type of this. 



