COURSE OF THE BUNDLES IN THE STEM. 2 75 



bundles in a transverse section of the internode is the same as in them. The lateral 

 bundles of the stem of P. crispus are however not cauline. The development of 

 their tracheae begins at the nodes, and proceeds from each of these towards the 

 middle of the next upper and lower internodes (Fig. 125, 4, 5). 



The course of the three bundles in the stem of Zostera marina ^ is just the same 

 as the above : one axile bundle is built up sympodially from the median bundles of 

 the leaves, while two lateral ones, which lie in a plane cutting the median-plane of 

 the bi-seriate leaves at right angles, give off lateral bundles to the leaves; these 

 however require further investigation. The arrangement of the bundles in the 

 transverse section of the internode of Zostera differs from that in Potamogeton, 

 since in the latter the lateral bundles are close to the central bundle, while in the 

 former they run at some distance from it, near to the surface of the stem. 



In Cymodocea aequorea Koen.^ the median bundle of the seven bundles of the 

 trace runs obliquely down to the middle of the stem, it there passes perpendicularly 

 to the next node, where it joins the median bundle, which there passes out. The 

 transverse section of each internode thus shows a central bundle. Near to the 

 periphery of the stem 20-25 small bundles (in weak stems fewer) arranged in two 

 concentric circles pass perpendicularly through each internode. Each of these, ac- 

 cording to Bornet, divides at the node into two, of which one ascends into the next 

 higher internode, the other either curves out into the leaf, or unites either with a 

 neighbouring peripheral bundle, or with the axile bundle. Besides this a complex net- 

 work of anastomosing bundles is formed in the node between the different bundles. 

 The peripheral ones appear to be cauline, but this requires to be further investigated. 

 As far as may be judged from transverse sections Cymodocea isoetifolia exactly re- 

 sembles other species. 



After what has been said ii need not be stated in detail how Hydrocharis and 

 Stratiotes might, as far as our present knowledge goes, belong equally well to this 

 as to the preceding section : this will have to be decided by further investigation. 



Sect. 71. In the foliage-shoots of Tamus and Dioscorea Batatas the vascular 

 bundles are arranged according to the Dicotyledonous type, that is in a ring sur- 

 rounding the pith. It is true that here the bundles pass an unequal distance into 

 the pith, but this also occurs in typical Dicotyledons. 



Nageli {/. c. p. 123) gives the following description of Dioscorea Batatas. The 

 leaves are sometimes spirally arranged, sometimes in decussating pairs. The leaf- 

 trace consists of three bundles. When the arrangement is decussate (Figs. 126, 

 127) its width is about 120°. If their course in a tangential direction be first con- 

 sidered (Fig. 126), the six bundles of one pair of leaves pass nearly straight down 

 two internodes, the lateral bundles {5c, ef\ hi, Im ; op, rs ; uv, yz) pectinating at the 

 first internode with the lateral ones which there enter. Above the bundles of the 

 second lower node the two lateral bundles of one trace converge towards one 

 another, and insert themselves on lateral bundles of the next lower pair; but the 

 median bundle {a, d; k, g \ /, x) divides into two shanks which unite with the same 



' Compare Magnus, Botan. Zeitg. 1872, p. 216. 



^ Boniet, Recherches sur le Phucagiostis major, Ann. Sci. Xat. 5 ser. torn. I. Compare 

 especially p. 39, pi. 6. fig. i, and pi. 11, fig. r. 



T 2 



