STRUCTURE OF COLLATERAL BUNDLES. 319 



According to the arrangement of the xylem and phloem, three main forms of 

 bundles are to be distinguished, which are designated the collateral'^, the concentric, 

 and the radial. One and the same bundle may at different points of its course pass 

 over from one to the other of these forms (cf. Fig. 147, 148). 



I. Collateral Vascular Bundles. 



Sect. 101. Collateral bundles are with rare exceptions characteristic of the stem 

 and foliage-leaves of the Phanerogams, also of the stem of Equiseta, Ophioglossese, 

 Osmunda, and Todea (?)''. Among parts belonging to the category of roots 

 they occur only in the tuberously developed roots of Dioscorese (D. Batatas), 

 Ophrydese, and perhaps those species of Sedum which are related to S. Telephium. 

 Cf. p. 233. 



In the most numerous and the typical cases they consist of a xylem and phloem 

 portion, each of which borders longitudinally on the other with one surface, and with 

 the remainder of its periphery on the non-equivalent surrounding tissue. A special 

 subordinate form, to be called the double collateral or bicollateral, is distinguished 

 from the usual one by the fact that two phloem groups lie on opposite sides of one 

 xylem group. These will be described last, and at present only the simple collateral 

 bundles will be discussed. 



The orientation of collateral bundles is in the usual cases, which we may call 

 normal, always such that the xylem is turned towards the middle, and the phloem 

 towards the periphery of the whole organ to which they belong. Accordingly we 

 can use the terms inside and outside as regards the bundle in a general sense, calling 

 the edge turned away from the phloem the inner edge, and using corresponding 

 words for the remaining sides. In the bundle-ring of the typically constructed 

 Dicotyledons and Gymnosperms (p. 235) all the xylem portions lie, in consequence of 

 the orientation mentioned, in an annular zone which immediately surrounds the pith, 

 while all the phloem portions occupy a zone concentric with the former and external 

 to it. The former, together with what is added later by secondary new-formation, is 

 traditionally known as the woody ring or woody mass, the second as the bast, bast- 

 ring, bast-zone or inner cortex, and the two parts of the bundle are accordingly called 

 the woody-part, and the bast-part or cortical-part — xylem and phloem ', the nomen- 

 clature originally adopted for the Dicotyledons being extended to the structurally 

 similar parts of all vascular bundles, without regard to arrangement and orientation. 

 The same orientation is also the rule for the stems of Monocotyledons, and for all 

 leaves or portions of leaves in which the bundles are placed in a ring around a 

 central part from which they are absent. Where on the other hand the bundles 

 in a leaf, or portion of a leaf, have an arrangement other than the annular one just 

 mentioned, their phloem is turned towards the morphologically lower leaf-surface, 

 and their xylem towards the upper, thus preserving the same orientation as in the 

 stem if referred to the latter, the leaf being supposed to be in the erect position. 



As regards Dicotyledons with the typical single ring of bundles, no exceptions 

 to these rules are known, unless perhaps in species of Strychnos (cf. Chap. XVI). 



' Riissow, Vergl. Unters. 



^ [Compare Haberlandt on collateral bundles in tlie leaves of Ferns, Bot. Ztg. 1881, p. 4S7 ; 

 idem, 1882, p. 217.] ' NageU, Beitr. I. 



