COURSE OF THE BUNDLES IN THE STEM. 



257 



Fig. 113. — Casuarina muricata. 

 Scheme of the course of the vascu- 

 lar bundles in the median longitu- 

 dinal section of a young branch. 

 I — 4 successive whorls of leaves ; 

 bundles of wI:orl, 2 ending in node 

 a, 3 in ^, 4 in c, &c. 



into the periphery of the stem, and there runs, parallel to it, in the cortex as far as the 

 next node : it then curves inwards, and ranging itself with those of the same whorl of 

 leaves around a narrow cylinder of pith, it descends per- 

 pendicularly through a second internode. At the lower 

 limit of this, i. e. at the 2nd node from the point of exit 

 into the leaf, it is inserted (according to Low with a short 

 fork) on the bundles which here pass out into the cortex. 

 The transverse section of each internode thus shows 2 con- 

 centric circles of an equal number of alternating bundles: 

 the one peripheral, composed of the bundles of the trace of 

 its own whorl of leaves, the other axile (forming the woody 

 ring at a later period), consisting of the bundles of the trace 

 of the next higher leaf. 



In Begonia angularis Raddi, Hildebrand^ found one bundle 

 in the outer cortex of each of the six angles of the stem. All 

 the six in one internode form together the trace of the next 

 higher leaf, which includes 4 of the circumference of the 

 stem. They pass perpendicularly downwards in the angles 

 to the next lower node, and here curve into the ring of 

 bundles. Their further course in the latter has not been 

 investigated. Many internodes have less than six angles, and 

 correspondingly fewer cortical bundles : others may be en- 

 tirely without either. The same is the case with Begonia 

 tomentosa, with this difference, that the number of the 



cortical bundles is 'indefinite' and often very large, while some of them often run 

 through two internodes in the cortex. 



Arceuthobhon Oxycedri - ha.- decussating pairs of leaves, each leaf has three bundles of 

 the trace, one median and two lateral. The latter converge, and enter the stem : here they 

 unite and descend opposite those of the other leaf of the pair, and separated from them 

 only by a narrow band of pith, to the next node, where they insert themselves upon the 

 bundles which there pass out. The thin median bundle of each leaf pursues an in- 

 dividual course through the cortex, and is also inserted at the next lower node. The 

 transverse section through an internode thus shows two decussating pairs of bundles, 

 one stronger axile pair, and one weaker and peripheral. 



In the Calycanthacex^ three bundles pass out into each of the opposite and decussate 

 leaves, one stronger median, and two weaker lateral ones. The median ones are 

 arranged in a ring in the stem. Each runs down through two internodes, and then affixes 

 itself at the node on the median bundles which there pass out. The lateral bundles 

 (rather later developed) pass perpendicularly down the stem in the cortex outside the 

 ring of bundles : at the next node they insert themselves on the cortical bundles which 

 there pass out. Thus in each internode the transverse section shows the ring of bundles, 

 and outside it four cortical bundles. In the node each cortical bundle is connected with 

 the ring by a short radial transverse bundle, and by another with the median bundle 

 which next passes out, and again, by a stronger horizontal girdle-like bundle, with the 

 next cortical bundle belonging to the same side of the stem. 



In the seedling two bundles enter each cotyledon : they descend with the median 

 bundles of the first three leaves through the hypocotyledonary axis, the transverse section 

 of which thus shows six bundles. The cortical bundles of the first two leaves only 

 extend down to the cotyledonary node. 



/ 



' I.e., compare p. 25.^. 



* (iraf zu Solms-Laubach, in Pringsheim's Jahrb. Bd. VI. p. 523. 



^ Mirbel, Ann. Sci. Nat. XIV (1828). — Gaudichaud, Archives de Botanique, II. p. 493 (1833). 

 -Woron'in, Botan. Zeitg. i860, p. 177. 



S 



