SCLERENCHYMA AND SCLEROTIC CELLS. 421 



the Pdtamogetons it has already been stated (p. 232) that they anastomose in the node 

 ■with one another and with the vascular bundles. Those in the cortex of Palms, 

 as already described at p. 266, after running through many internodes, are sometimes 

 prolonged into purely fibrous bundles, which pass out into the leaves, and sometimes 

 pass over into the fibrous investment of vascular bundles, which likewise make their 

 exit into leaves. Connected with this is the phenomenon likewise mentioned above, 

 of the occurrence, in the plants last named, of intermediate forms between purely 

 fibrous strands and those which contain small complete vascular bundles or single 

 sieve-tubes. 



It may here be the best place to recall to mind the fibrous strands which sur- 

 round a secretory passage in the leaves of Pinus and the roots of Philodendron. 

 Comp. p. 202, Fig. 185, p. 381 ; Fig. 168, p. 360. 



The phenomena hitherto described show that the arrangement of fibrous strands 

 and fibrous sheaths is in a high degree independent of the course of the vascular 

 bundles ; but that, on the other hand, there are also close relations between the two. 

 A further phenomenon, corresponding to the attachment of the vascular bundles in 

 stems to the rings or sheaths described, or their inclusion in the latter, occurs widely, 

 especially in the Monocotyledons mentioned above, inasmuch as the vascular bundles 

 are attached to the inner edge of the hypodermal fibrous ridges which project in- 

 wards, or are included in the latter. In flat leaves with fibrous ridges passing verti- 

 cally through them from one side to the other, the latter often have a bundle inserted 

 in the middle, or several one above another near the middle. Side by side with these, 

 other ridges or strands often occur, at least among Monocotyledons, which are quite 

 similar to the former in structure, but contain no vascular bundles, whether they are 

 so far related to one of the latter that they stand opposite to it, or whether even 

 this relation is absent. 



In so far as the vascular bundles are attached to or included in those of the 

 sclerenchyma, the latter stand to the former in the relation of sheaths. The same 

 relation, as already stated above (Sect. 99 et sq.), is also of general occurrence in the 

 case of those vascular bundles which are not attached to continuous sclerenchjnma- 

 tous sheaths or to hypodermal strands; the sclerenchymatous fibres follow their 

 course as bundle-sheaths (p. 318), which may serve both as a local protective ap- 

 paratus for the single' bundle, and as a strengthening apparatus for the entire organ. 

 Together with the vascular \<niidSes,\h'ij ioxxn fibr avascular bundles (p. 318). Sclerotic 

 endodermis may take part in this function, as was stated in former paragraphs. 

 Between sheaths which simply follow the bundle, and attachment to sclerenchymatous 

 bundles the position of which is otherwise determined, the most various intermediate 

 forms occur, as shown in detail by Schwendener. 



Apart from these relations of position, the fibrous sheath of the bundle is either 

 closed all round, or partly interrupted, or only partial, i. e. limited to a relatively small 

 portion of the circumference. The first-mentioned relation of complete sheathing 

 obviously occurs in those of the cases above-mentioned, where the bundles are com- 

 pletely inserted in a closed ring of sclerenchyma. It further exists in other forms 

 of insertion or attachment, and in the case of individual fibro-vascular bundles, 

 especially, but -not exclusively, in Monocotyledons. In isolated collateral fibro- 

 vascular bundles the fibrous sheath is then rarely of approximately equal thickness 



