CHAPTER III. — TISSUE SYSTEMS. 



177 



bundles found in the rhizomes of some Monocotyledons, the 

 reverse order prevails, — that is, the xylem surrounds the phloem. 

 Fig. 437 represents the concentric bundle of Pteris aquilina, a 

 Fern. The large scalariform ducts occupy the interior of the 

 bundle ; the sieve and other phloem tissues are exterior to these, 

 and the whole is surrounded by the endodermis. 



(c) The Radial Bundle. In this form of bundle the xylem 

 tissues are arranged in radial bands or plates, and each is sepa- 

 rated from the next one by a mass of phloem tissues. In Fig. 435 

 there are four radiating masses of xylem alternating with as 

 many of phloem. The whole is surrounded by a bundle-sheath 



♦37 



Fig. 438. 



Fig. 437. — Concentric fibro-vascular bundle of Pteris aquilina: a, endodermis: b, large 

 scalariform duct in xylem ; c, phloem tissues surrounding the xylem. Magnified about 150 

 diameters. 



Fig. 438. — Radial fibro-vascular bundle from the root of Acorus Calamus; a, endo- 

 dermis; b, pericambium: c, phloem mass; d, xylem ray, composed mainly of ducts, the 

 outer ones of which are thick-walled, and the inner ones larger, but thinner walled; e, cen- 

 tral parenchyma. Magnified about 125 diameters. After DeBary. 



consisting of rather thick walled cells. Sometimes the bands of 

 xylem meet in the centre, and in other cases they are connected 

 by a central cylinder of parenchyma, forming a kind of pith. 

 Radial bundles occur in most roots, and in the filiform stolons of 

 Nephrolepis, a Fern. In the section of the fibro-vascular bundle 

 of the root of Acorus Calamus, represented in Fig. 438, which 

 may be taken as typical of the kind, the whole bundle is seen to 

 be surrounded by an endodermis ; interior to this is a layer of 

 active, thin-walled cells, called the pericambium, from which new 

 roots originate ; plates of tracheary tissue farther interior alter- 



