320 PRTMARV ARRANGE^fENT OF TISSUES. , 



In stems, leaf-stalks, and leaf-ribs with several concentric rings of bundles, or 

 bundles scattered in cross-section, exceptions certainly occur, though rarely; also 

 ver)' rarely in the lamina of flat leaves. 



In the positions first mentioned precisely the opposite orientation to that in the 

 normal case is shown by certain bundles in the stem of Nelumbium (Fig. 1 1 2, p. 255), 

 namely by those of circles 3 and 5 in the intermediate series ; further by the 

 medullary bundles in the stem of the Aralias mentioned on p. 253, by the four 

 cortical bundles in the internode of Calycanthus, by those of the middle one of the 

 three concentric bundle-circles in the leaf-stalk of the Lime \ and several others. 



In stems and leaf-stalks many bundles which are scattered as seen in cross-section 

 have an irregular orientation, i. e. with the two parts facing neither directly outwards 

 nor directly inwards. Those especially which anastomose or branch often show 

 torsions which divert them from the normal orientation, near the points of branching 

 or of union. Examples of this are found in many leaf-stalks, e. g. Aralia japonica, 

 Aroideoe ; in the pith of Silaus and other Umbelliferse mentioned at p. 253 ; and in 

 the interior of the stem of Aroidese and Pandaneae (cf. p. 268). 



In the leaves of Typha and Agave Americana (p. 305) the bundles running 

 through the middle layer, which is destitute of chlorophyll, all have their vascular 

 part turned towards the upper side ; in those on each side which border on the 

 chlorophyll-parenchyma the vascular part faces the middle of the leaf. Of the 

 longitudinal bundles lying in one plane in the leaf-lamina of Dracaena reflexa, the 

 median one has normal orientation, while all the rest have their xylem turned towards 

 this, and their phloem towards the edge of the leaf. 



The general form of the cross-section of collateral bundles is as a rule round 

 or oblong; in the latter case the greater diameter passes as a rule through the 

 middle of the outer and inner edges. In the stems and leaves of several Mono- 

 cotyledons this unequal extension on different radii of the cross-section amounts to a 

 marked lateral flattening of the bundle ; e. g. leaves of Scitaminese, Asphodelus 

 luteus, Hemerocallis, Hyacinthus, Pandanus, leafy-stem of Canna, &c. Other shapes 

 are rare ; such as the horseshoe-shaped cross-section of the bundles in the stem of 

 Osmunda (Fig. 128, p. 280), the annular section in the stem of Botrychium (p. 284), 

 and in the petioles and ribs of several Dicotyledonous leaves, as those of Eriobotrya 

 japonica, Veronica speciosa^, the pulvinus of the leaf-stalk of Mimosa pudica, &c. 

 In these latter cases the inner edge of the ring is always the inner edge of the 

 bundle, not only as regards orientation, but also as regards structure. 



Ignoring these annular bundles, and ignoring the places where several bundles 

 meet, which we shall describe below, both the whole collateral bundle and each of its 

 two parts form an approximately, though never exactly monosymmetrical body, with 

 its plane of symmetry passing through the middle of its outer and inner edges. The 

 arrangement of the elements in the bundle is also in harmony with this approximate 

 monosymmetry, as will be shown below. 



The number of elements in each part, and the resulting thickness of the bundle, 

 is extremely different according to the individual cases : many sappy herbaceous 



' Compare Frank, Botan. Zeitg. 1864, p. 381. 



^ Areschoug, Om bladets inre byggnad. Lund's Univ. Arskrift, torn. IV. 



