3T4 PRIMARr ARRAXGEMEXT OF TISSUES. 



Of the Ferns with divided upper and lower bundles, which are connected by 

 intermediate forms with those above described, and with those in which there is 

 only a complex, often irregular network of bundles instead of two bundles (see 

 p. 288), this only may be stated generally, that in many simply constructed forms, 

 e. g. Polypodium squamulosum, the bundle-system of the lateral shoots arises as a 

 simple bundle from a definite mesh in that of the main axis. In the large majority 

 of these cases several thin bundles, which enter the lateral shoot, arise from the 

 margin of definite meshes (comp. Fig. 136, p. 287). Their number varies according to 

 the species, from two to eight, as far as present data extend, and is always smaller than 

 that which enters a leaf of the same species. Where medullary bundles are present, 

 and there is continuity between the pith of the main and lateral shoot, branches 

 separate from the medullary bundles of the former and enter the latter, e. g. Polybo- 

 trya Meyeriana; where the bundles of the lateral shoots insert themselves as a simple 

 not hollow bundle, such a branching does not occur. 



When the lateral shoots arise from the base of the leaf, modes of insertion of 

 bundles are found which are similar and subject to similar variations to those of the 

 bundles arising from the stem : this is shown by the above-mentioned example of 

 Aspidium Filix mas. There are but few thorough investigations on this point. 



No exact investigations have been made either on the insertion of the bundles 

 at the rarely occurring points of branching of the Osmundacese, or on the mode of 

 origin of these branchings. 



In theEquiseta the bundle-systein of each branch is united into a single bundle, 

 and inserts itself externally at the node of the main shoot on the angle of branching 

 of one of the bundles descending from the next higher leaf-sheath (comp. p. 279)^ 



Where the branching appears and remains as a forking of the main axis, the 

 whole bundle-system also divides into two parts, each of which enters into one 

 branch of the fork : both systems are fundamentally similar to one another and to 

 that of the main axis. 



Among Ferns the rhizomes of Pteris aquilina ^ are — if we disregard for the 

 present the above-mentioned controversies — an exquisite example of this. Comp. 

 Fig. 143, p. 295. Also in Athyrium Filix foemina, according to Hofmeister, the phe- 

 nomenon is frequent; in Aspid. Filix mas it is rare; these cases are regarded, it is 

 true, by Mettenius as an apparent forking, derived from early stronger development 

 of monopodially formed lateral shoots. 



In the Lycopodia and Selaginellae, whose branchings are always forked — though 

 not always quite equal — the insertion of the vascular bundles which enter the branches 

 is generally indicated by what has been said above. In the Selaginellae with two 

 lateral bundles in each shoot, as in S. Kraussiana, INIartensii, &c. (comp. p. 282), of 

 the four bundles destined for each branching either three diverge acutely from one 

 bundle of the main shout, and the fourth is the continuation of the other which 

 traverses the main shoot; or each of the latter divides into two branches which are 

 very close to one another at their entry into the fork. In S. Martensii the former 



' See Stcnzel, /. c, Taf. IV. Fig. 13. * Compare Hofmeister, Stenzel, //.cc. 



