372 



PRTMARF ARRANGEMENT OF TISSUES. 



The dilatation of the terminal branches above-mentioned comes about either 

 by dilatation of the individual tracheides, or by increase in the number of rows. 

 The end surfaces of the tracheides, bordering on the parenchyma, are usually cut off 

 sharply, either transversely or obliquely. Characteristic differences corresponding to the 

 several main forms of distribution of the bundles, or to the larger systematic divisions, 

 are not to be observed, except that in general the thickness of the single terminal 

 branches diminishes where the branches are numerous. Thus in the Ferns investi- 

 gated, the relatively few final ramifications are comparatively thick, consisting of 

 several rows, while in the foliage of Dicotyledons, where the reticulations are 

 abundant and internal ends are present, the bundles are at last, as it were, broken up 

 into single, or in places double tracheal rows, which end free with short branches 

 (Figs. 172, 173). The transverse branchlets of most Monocotyledons consist of 



J^IG. 172.— Psoraleabituminosa(4o). Ultimate FiC. 173 (225). — Ultimate ramifications of vascular 



1 ramifications of the bunciles in a piece of a bundles from the leaf-lamina of Psoralea bituminosa ; 



leaflet ; at ?- is the edge of the latter. branched rows of tracheides, the ends marked x being 



torn off, the others terminating free. The whole 

 branched row is surrounded by large cells containing 

 chlorophyll ; outside these are the circular transverse 

 sections of some cells of the dense palisade parenchyma 

 of the leaf. 



one, or of quite a few rows of tracheal elements (Figs. 174, 175). The leaf of 

 Welwitschia is to be mentioned as an exception to the rule, for its very numerous 

 transverse branchlets (comp. Fig. 145, p. 303) have, so far as can be determined, the 

 structure of complete vascular bundles provided with a multiseriate xylem and 

 phloem, and it is only the short, thick, free-ending branches, springing partly from 

 the angles of the cross-branchlets and partly direct from the longitudinal bundles, 

 which consist exclusively of tracheides, the latter being inserted between the elements 

 of the surrounding parenchyma. 



As follows from what has been stated, the ultimate vascular branches often 

 border directly on the parenchyma which in the laminae of foliage-leaves contains 



