344 



PRIMARr ARRANGEMENT OF TISSUES. 



and pass down the corners, between which the leaf is placed, through two inter- 

 nodes. At the second lower node they unite with those of the next lower trace, after 

 having traversed one internode close side by side with these. The transverse section 

 below the apex of the stem thus always shows eight bundles, in groups of two beneath the 

 corners ; those of each pair are of unequal size, the stronger belonging to the leaf-trace 

 of the nearer pair of leaves, the weaker to that of the next pair. The bundles of one 



corner soon unite, as vessels appear between theft. 

 The transverse section then shows four bundles, which 

 later unite to form a closed ring (Chap. XIV). 



19. Leaves opposite, traces of 3 bundles : the lateral 

 bundles pectinating with those of the next pair. 



Clematis Vitalba, Viticella ; Atragene, Urtica Dodartii, 

 Lonicera spec, Acer pieudoplatanus, Philadelphus coro- 

 narius,Tagetes lucida, T. signata Bartl., Humului Lufu- 

 lus, Centranthus ruber, jEsculxu macrostachya, Euphorbia 

 Lathyris. 



The foliage shoots of the above plants, though 

 corresponding in the above points, differ in the unequal 

 length of the course of the traces. The median bundles 

 sometimes insert themselves at the next lower node, 

 sometimes at the next but one, sometimes still lower. 

 The lateral bundles also pass through one, two, or 

 several sections of the stem. Giving the reference to 

 Nageli's work, we will here describe only the very 

 simple examples of Clematis and Atragene (Figs. io6, 

 107). 



The pairs of leaves decussate at right angles. The 

 six corners of the internodes, of which two opposite 

 ones corresponding to the median points of the leaves 

 are rather more prominent, alternate regularly. The 

 width of the three-bundled leaf-trace is about 115°. 



The median bundles {ad, gk, qn, xt) pass through 

 one internode, divide at the next node into two shanks, 

 and insert themselves with these on the lateral bundles 

 of the pair of leaves at that point. At first there is 

 always but one shank present, and the two median 

 bundles of the same pair have (according to two 

 observations) a symmetrically converging curvature. 

 The formation of the second shank often appears at a 

 late stage in CI. Viticella, or is entirely absent. 



The two lateral bundles of the leaf {be, ef, hi, 

 Im, Sec.) also pass through one internode, they curve 

 in a converging manner at the next node, and insert 

 themselves on those same lateral bundles, with which 

 the shanks of the median bundle unite. In CI. Viticella 

 the leaf-trace in this condition is usually complete : in 

 CI. Vitalba a second shank is formed at the point of 

 curvature of the lateral bundles also : this curves to 

 opposite side, and coalesces with a median bundle of the node. The trans- 

 verse section of the young internode shows six bundles of the leaf-trace (Fig. 107, 

 p. 246). 



The axillary branches have also six bundles in their lowest internode, which unite to 

 two on entering the stem. These two unite at once right and left with the median 

 bundle of the leaf which bears them. 



Fig. 104. 



Fig. 105. 



FIGS. 104 and log.— Stachys angustifolia, 

 after Niigeli. Fig. 104. Scheme of the vascu- 

 lar system in the end of the shoot, the cylin- 

 drical surface being exposed in one plane. 

 ab, dc^fe, £k, zA, the traces of successive 

 pairs of leaves, the letters being placed at the 

 nodes ; from the highest pair only one bundle 

 of each trace is as yet visible, zA — Fig. 105 

 (35). Transverse section through a young in- 

 ternode, corresponding to that above a 6 in 

 the above figure ; the same bundles are indi- 

 cated by the same letters as in the above. 



the 



