236 PRJMARy ARRANGEMENT OF TISSUES. 



the ring — this is the pi//i or Medulla ; a mantle, covered by the epidermis, and sur- 

 rounding the ring externally — this is the outer cortex ; and the bands which lie be- 

 tween the bundles, and pass radially, as seen in transverse section, from the cortex to 

 the pith — these are the primary connections with the pith, ox primary medullary rays. 

 The form and number of the latter is defined in each individual case, in the first 

 instance, by the above-mentioned general rules for the number of bundles, and for 

 the course of the leaf-traces : their form depends upon the relative width of the 

 vascular bundles. 



In the following paragraphs will be given the most important known individual 

 cases, following in the main the fundamental investigations of Nageli, while the 

 most prominent general rules will be more thoroughly exemplified by some few 

 instances. 



A. DlCOTYLErONS, 



I. Hypocotyledonary stem. In most species investigated two bundles of the trace 

 enter the hypocotyledonary stem from each cotyledon : these usually unite at the base 

 of the cotyledon as its central nerve {e.g. Plantago, Urtica, Mercurialis, Antirrhinum, 

 Impatiens, Troparolum, Vitis, Lupinus, Lathyrus, &c. ^). In Phaseolus the two bundles 

 are sometimes separated, sometimes united. In many plants the trace of the cotyledons 

 consists of but one bundle {e.g. Papaver orientale, Lepidium sativum, Spergula arvensis, 

 Silene'^, Portulaca oleracea, &c.), though it is possible that here also this often arises 

 through the very early coalescence of two bundles. In Cucumis sativus and Melo 4, and 

 in Mirabilis Jalapa 5 bundles enter the cotyledons, in Ricinus communis 4 or 5. From 

 the cotyledonary node the bundles run vertically downwards, and unite at the base of 

 the hypocotyledonary part. Where the trace of the cotyledons is a single bundle these 

 remain throughout separate and undivided. Where it consists of two bundles their 

 behaviour is not uniform. Either the two bundles of one trace approach one another, 

 and finally unite as a single bundle ; or the non-equivalent bundles of the two traces 

 unite, the right-hand bundle of the one with the left-hand bundle of the other. A 

 transverse section shows in the first case two bundles, the position of which corresponds 

 to that of the cotyledons (Lupinus luteus, Lathyrus Aphaca, Urtica Dodartii) ; in the 

 second case two bundles alternating with the cotyledons {e.g. Antirrhinum majus, Tro- 

 paeolum majus, Impatiens Balsamina, Vitis vinifera). The four-bundled traces of the 

 cotyledons in Cucumis are united at their margin, since the lateral bundles from 

 the respective cotyledons coalesce with one another— thus the transverse section of the 

 hypocotyledonary stem shows 6 bundles, 4 separate and 2 united : the two latter 

 separate again at the base into two shanks, and unite with the first four. The 8, 9, 

 or 10 bundles, which enter the cotyledons, unite in Ricinus to 4, in Mirabilis to 2. 



The bundles, which enter from the leaves next above, insert themselves in the cotyle- 

 donary node on those of the cotyledons, or run down into the hypocotyledonary stem, 

 and finally unite with them there: the latter is the case e.g. in Lupinus and Phaseolus 

 (see below under 4). 



II. Region of Foliage. 



I. Leaves arranged spirally, leaf-trace a single bundle. 'The bundles from the leaves 

 descend through many internodes, and usually unite with those of definite lower leaves, 

 so that a transverse section shows the traces in a definite spiral series, which is not 

 identical with the spiral of the leaves, but is related to it,' 



' Niigeli, I.e. p. 61. — Lestiboudois, Phyllotaxie anatomiqne, Ann. Sci. Nat. 3 ser. torn. X. 

 p. 19. 



■^ Rohrbach, Monogr. d. Gatlung .Silene, p. J3. 



