2^6 PRIMARY ARRANGEMENT OF TISSUES. 



three between each two pairs of the above • the bundles of the next (9) alternate with those 

 of (8), those of (10) and (11) alternating irregularly with those next within them. 



Of these bundles all those of the peripheral series are, according to Wigand, 'true 

 leaf-bundles, since they traverse only one internode and then run into the leaf-organs.' 

 . The course of the rest is difficult to describe with certainty, because of the complicated 

 branchings of lateral shoots and roots in the node, and it requires further investigation. 

 The bundles of the inmost circle of the middle series (3) seem to be cauline, since ' it 

 has not hitherto been possible to prove that they contribute to the lateral organs,' and 

 they ' apparently always traverse one internode only, and lose themselves in the node, 

 their place being taken by fresh ones in the following internode.' The same holds 

 perhaps for the other members of the middle series, which are radially arranged. Of 

 the inmost series the four lateral bundles of the inner side (i) give otf branches to the 

 roots, the upper and under ones of the same circle give off branches ' directly or in- 

 directly to the leaves. All six bundles of this circle 'are however distinguished by 

 the fact that they alone of all the bundles of the stem traverse all internodes and nodes 

 up to the growing point,' while on the contrary the six bundles of the same series, which 

 alternate with them (2), ' each belong to one elongated internode only, and to the roots 

 (which arise at each node), and then end in the node, just above the root-region.' 



The medullary bundles of the Melastomaceis will be described below, p. 259, in con- 

 nection with the rest of the bundle system of these plants. 



b. Cortical bundles. 



Sect. 63. A relatively small number of Dicotyledons is characterised by having 

 a bundle system in the internodes arranged typically in a ring, and outside this other 

 bundles, which traverse the cortex. These cortical bundles are sometimes bundles 

 of the leaf-trace, which run for a certain distance outside the ring, and later curve 

 into it : as in the cases of Lathyrus Aphaca, and Pseudaphaca above described, the 

 Casuarinas, and many Begonias : also the cortical bundles of the Cycadeae, to be 

 described in detail in Chap. XVI, belong to this category, perhaps also Nepenthes. 

 Sometimes they are certain bundles, belonging to many-bundled leaf-traces, which 

 never enter the ring, but form with the similar ones belonging successively to upper 

 and lower leaves an independent cortical bundle-system only connected with the 

 ring by anastomoses at the nodes : this is the case in the Calycanthese, many 

 Melastomaceae, also in Arceuthobium Oxycedri. In many succulent plants with 

 reduced leaves, as Salicornia, Cactese, they are branches of the bundles of the 

 leaf-trace, which are branched and distributed like the bundle-expansions of the 

 leaf lamina, and will therefore be treated of where these expansions are described. 

 Finally, in the winged Rhipsalidacese, according to Vochting, the peculiar case occurs 

 that the bundles of the leaf-trace are mainly cortical, while a ring of bundles, 

 surrounding a pith, and quite similar to the typical Dicotyledonous ring of bundles 

 of the leaf-trace, consists at least for the most part of cauline bundles. 



The young foliage shoots of the Casuarinas^ (Fig. 113) are covered with whorls of 

 small leaves, united into a long sheath : the average number of leaves of one whorl 

 varies according to the species (4-20). The leaves of successive whorls, and the 

 ridges, which run down the backs of the leaves, alternate in successive internodes. One 

 vascular bundle enters each leaf. From the point of insertion of the sheath it passes 



' Compare Low, De Casuarinearum caulis foliique evolutione et structura, Berlin, 1865. 



