2^2 PRIMARY ARRAVGEMENT OF TISSUES. 



of sclerenchyma, either singly or 2-3 together, running longitudinally through the 

 internodes, and often anastomosing at the nodes with the sieve-tubes of the vascular 

 bundles. Tr^cul describes small bundles of sieve-tubes, accompanied by laticiferous 

 tubes, distributed in the peripheral cortex of Gundelia Tournefortii. Also the bundles 

 described by Sanio, /. c, in the cortex of Plantago and Trientalis perhaps belong to 



this category. 



In many species of Potamogeton (P. natans, lucens, pectinatus) there is in many, 

 but not all of the bundles of sclerenchymatous fibres which traverse the cortical 

 parenchyma a small strand consisting of a few tubes, which is surrounded by the 

 sclerenchyma as by a sheath (corap. Fig. 171). With the above may perhaps be 

 grouped those bundles which can hardly contain sieve-tubes, found by Sanio ' in 

 the cortex of Elodea. Near to the Epidermis there are in the internode 6 bundles 

 of some few (usually 5) thin-walled, elongated-prismatic cells, which alternate with 

 the 6 rows of leaves. They run perpendicularly through the internode, and each 

 gives off on each side at each node a horizontal branch, which anastomoses with 

 the rudiment of a vascular bundle on its course into a leaf. 



2. Vascular bundles. 



Sect. 58. From the very first those bundles which consist essentially of de- 

 finitely arranged groups of tracheae and sieve-tubes, and which traverse the body 

 of the plant as a continuous system, with blind endings only in the growing-points 

 and at the ends of peripheral branches, have been called Vascular bundles, Fasciculi 

 vasorum. Inasmuch as the vascular bundles are not unfrequently accompanied by 

 sclerenchymatous fibres, the term Fibro-vascular bundles has in recent times been 

 frequently applied to them ^. 



The general arrangement of the tracheae and sieve-tubes, which are united to 

 form the bundles, is defined partly by their arrangement in the single bundle, partly 

 by the arrangement or course of the bundles in the plant. The former, that is the 

 structure of the single bundle, may, as is shovyn by experience, change in diff'erent 

 parts of its gourse. A synoptical description of the structure of the individual 

 bundles must therefore presuppose a knowledge* of their course, and the general 

 description must deal with this first. 



A. ARRANGEMENT OF THE VASCULAR BUNDLES. 



a. Course of bundles in the root. 



Sect. 59. In the individul root a bundle which terminates at the growing-point, 

 and which grows with it, runs almost exactly along the longitudinal axis ; in Isoetes ' 

 it is strongly excentric, and nearer to that side of the root which is opposite the 



' Sanio, Botan. Zeitg. 1865, pp. 186, 191. 

 " Nageli, Beitr. I. 



' Compare Von MoM, Linnaea, 1840 ; Verm. Schriften, p. 122, &c. — Hofmeister, Abhandl. d. 

 Kv Sachs, Gesellsch. d. Wissensch. IV. p. 147. 



