422 PRIMARV ARRANGEMENT OF TISSUES. 



all round, e. g. in the rhizome of Carices. In the majority of cases it is thicker on 

 the outside, where it embraces the phloem, than on the inner border of the bundle; 

 the converse relation occurs more rarely, e. g. in the rhizome of Scirpus palustris, in 

 the periphery of the stem of Saccharum officinarum, Bambusa spec, and other 

 species described by Schwendener. And further its thickness usually diminishes at 

 the lateral edges of the bundle, so that here, especially next the limiting surface of 

 xylem and phloem, it is often only 1-2 layers of fibres in thickness, while at the 

 outer or inner edge its thickness amounts to many layers. This is the case, for 

 example, in most bundles of Acorus and Zea (comp. Fig. 147, p. 317; Fig. 150, 

 p. 330), and many other IMonocotyledons ; in the stem of Saururus and its allies, in 

 the leathery leaves of species of Melaleuca, Eucalyptus, Eugenia, Callistemon, &c. 



To the latter condition is related the most frequent form of local m/er- 

 ruptio7i of the fibrous sheath, which consists in the presence of a gap of greater 

 or less extent, filled up by comparatively thin-walled parenchyma, next the lateral 

 edges of the limiting surface of phloem and xylem. Such gaps, or ' avenues ' as 

 Schwendener calls them, from the surrounding parenchyma to the vascular bundle, 

 are a widely distributed phenomenon in the region indicated, among the bundles of 

 tough parts of Monocotyledons, e. g. in the stem of Canna, the leaves of Typha, 

 Musa, Yucca, and Phormium spec. The bundle of Ranunculus repens represented in 

 Fig. 152, p. 331, is a good example of this, as is also that of Welwitschia, Fig. 157, 

 P- 335 '•> i-he same condition obtains in the bundles of many tough Dicotyledonous 

 leaves, e. g. species of Hakea and Lomatia. Lateral avenues also appear to occur 

 occasionally in the leaves of the INIyrtaceae mentioned above. 



In a species of Bambusa investigated by Schwendener, an avenue exists on the 

 inner side of the internal bundles of the stem, in addition to the two lateral ones. 

 The fibrous strand, which is of immense thickness on the inner edge, is here divided 

 by a transverse lamella of parenchyma into a narrow, thicker-walled section bordering 

 directly on the xylem, and a broader, thinner- walled peripheral section. The former is 

 usually interrupted by two short bands of parenchyma, which lead from the transverse 

 lamella to the xylem. 



The partial fibrous sheath of collateral vascular bundles usually occurs in such 

 a form that the phloem is supported in its whole extent, or only at its outer edge, by 

 a more or less strongly developed fibrous mass, often only by a small group, or even 

 by single scattered fibres. This is the prevailing rule in the leaves and stems of 

 Dicotyledons (Figs. 154, 156, pp. 333 and 334), and is besides not uncommon in 

 ^Monocotyledons, e. g. in the leaf of species of Crocus, Agave, and Dracaena, in the 

 sheath of the leaf of Zea (Fig. 151, p. 331), and in the stem and petiole of Aroidese, 

 as Arum and Colocasia. Comp. also the small bundles of Acorus, Fig. 147, p. 317. 



The converse condition, that the partial fibrous sheath embraces the inner edge 

 of the xylem, is more rare : it occurs in the smaller bundles in the periphery of the 

 halm of Papyrus, the halm of Cyperus vegetus and other Cyperacese (comp. Schwen- 

 dener, /. c). 



The fibrous strands which appear in company with the vascular bundles often 

 occur, as shov. n by the examples of Grasses and Cyperaceaj adduced above, side by 

 side with those otherwise arranged, but in very many cases they are present alone. 

 The latter holds good for most of the Dicotyledons which are not expressly 



