ENDS AND CONNECTIONS OF THE BUNDLES. 37 1 



The former process goes on as a rule more slowly in the roots, the vessels persist 

 longer, than in the stems of the same plants. The structure of the root-bundle (peri- 

 cambium, alternating xylem and phloem rays) here preserves its typical character, 

 although the number both of the individual rays, and of the elements composing each of 

 them, is low, the former being reduced to four or to two, the latter often to one. The 

 mature root has therefore the typical bundle-structure, with the exception that in the 

 place of the 2-4 xylem rays, each of which is represented by a vessel, and in place of 

 the large central vessel, which in many cases unites the rays, an intercellular passage is 

 present; e.g. Aponogeton, Alisma, Hydrocleis (van Tieghem). 



In the root of Elodea canadensis, as in the stem of the same plant, the disappearance 

 of the 4-5 peripheral vessels, and of the larger central vessel, which is separated from 

 them by an annular layer of parenchyma, takes place immediately after their first, 

 incomplete formation. 



Among those root-bundles which, on the other hand, remain rudimentary, those of Najas 

 belong to this category. They consist of two layers of elong ited delicate cells, and 

 thesQ surround an axial passage, which arises by the ab-orptioa of a row of meristematic 

 cells. 



In the root of Vallisneria, according to van Tieghem, there is only an annular layer of 

 elongated cells enclosing an axial passage, and surrounded by the endodermis, and this 

 constitutes the rudiment of the bundle. The delicate bundle in the root of species of 

 Lemna shows in cross-section essentially the same structure ; the middle is according to 

 Hegelmaier occupied by a row of cells (not by a passage). Spirodela polyrrhiza shows the 

 same character, but with the difference that the row of cells in the middle is developed 

 into a persistent row of narrow annular tracheides. 



II. Ends and Connections of the Vascular Bundles. 



Sect. iir. The ends of the vascular bundles, as was shown above at Sect. 91, 

 lie in the foliar expansions, and in the corlex of many plants, either as internal ends 

 ceasing in the parenchyma, or forming anastomoses, or as peripheral ends at the 

 edge or surface of the leaves; in the stems of Cyatheacese, described at p. 291, they 

 also end in the interior of the pith. 



With the ultimate degrees of ramification the thickness of the bundles usually 

 diminishes, both the number and the size of their elements being reduced ; at the 

 extreme free ends they often, but not always, show a terminal dilatation. Xylem and 

 phloem do not behave alike in this respect. Clearly characterised sieve-tubes are, 

 it is true, often still present in the thicker bundles of the foliage leaf, e. g. in those of 

 the leaf-nerves ; in the last orders of ramification they are no longer to be found, 

 the latter consist either of tracheae alone, or of these and of delicate elongated cells 

 accompanying them, the sieve-tube nature of which is no longer recognisable. 

 Where and how the sieve-tubes cease and end has not hitherto been clearly made 

 out in any case, and this point deserves more accurate investigation. The tracheae 

 always form the direct continuation of the xylem of the thicker bundles. 



The ultimate internal ends and anastofnosing branches of the bundles consist 

 only of one or a few rows of short tracheides with fine spiral thickening, or reticulate 

 thickening with narrow transverse meshes ; their wall is often quite smooth in parts, 

 appearing as if immature, e.g. in the leaf of species of Chamaedorea, and Zea Mais 

 (Fig. 175). Whether vascular perforations occur in these rows is at least doubtful, 

 and not easy to decide. 



B b 2 



