362 PRIMARY ARRANGEMENT OF TISSUES. 



the Pandanes, for example, I find them, as seen in cross-sections, isolated and irregularly 

 distributed, in the thickest roots of Freycinetia nitida of the Berlin Gardens; in Pandanus 

 pygmsus (graminifolius of gardens) they are arranged in transverse rows (parallel to a 

 diameter), which are separated from one another by broader bands of parenchyma. In 

 P. odoratissimus, two or more strands separated by narrow bands of parenchyma are 

 placed together in groups, and the groups scattered between broader masses of paren- 

 chyma. As the thickness of the roots diminishes the conditions of structure described 

 become simplified. A branch-root of Pandanus pygmaeus, 1-2 mm. in thickness, has, for 

 example, inside the radial ring about 2-3 large vessels, and the same number of phloem- 

 groups, enclosed in homogeneous fibrous sclerenchyma, which is directly continued into 

 the ring. Branch-roots of Dracaena reflexa about i mm. in thickness have a thoroughly 

 typical structure, the radial ling surrounds a thin-walled axial cylinder of parenchyma. 

 It is only in thicker roots that an irregularly placed strand of sclerenchyma containing 

 vessels appears, such strands becoming very numerous as the roots increase in thickness. 



The roots of Iriartea, finally, which are an inch in thickness, are distinguished from 

 those last described, first by the fact that their bulky vascular mass is not cylindrical, but 

 deeply furrowed, having in cross-section the foi m of a star with about ten blunt and 

 usually bifid rays ; further by the fact that the radial ring also is divided up into scleren- 

 chymatous bundles, enclosing the vessels and phloem-groups, and radial bands of paren- 

 chyma, which are sometimes narrow, 1-2 layers in thickness, sometimes many-layered, 

 and which separate the bundles from one another. The middle of the star also consists 

 mainly of thin-walled parenchyma, often with lacunae, which is directly continued into 

 the radial bands of the ring, and in which bundles of sclerenchyma, each containing one 

 or more vessels and phloem-groups, lie scattered. Inside each sclerenchymatous bundle 

 the vessels are surrounded by 1-2 layers of parenchymatous cells, those of them which 

 belong to the ring standing in direct connection with the many-layered pericambium. An 

 endodermis, which is thickened here and there, appears according to Mohl's figure to 

 surround the star. Finally, in the entire parenchyma, both of the star and of the cortex 

 which surrounds it, numerous small bundles of Eclerenchymatous fibres lie, each enclosing 

 in its centre 1-2 thin- walled elongated elements (perhaps sieve-tubes?). The xylem- 

 plates in the ring appear short and irregular in cross-section, their radial arrangement and 

 alternation with the phloem-plates is according to Mohl's figure often indistinct, though 

 in general to be recognised. The development of the elements, both in Iriartea (Karsten) 

 and in the roots of Pandanus, begins at the periphery of the ring, and in general proceeds 

 centripetally. According to all these phenomena, the series of large roots just described 

 are immediately connected with the type of Monocotyledons as special cases, in which 

 the anatomical differentiation becomes more varied, with the more considerable size. 



The system of bundles, which traverses the tuberous roots mentioned at p. 233, is 

 entirely different in structure from the bundles last mentioned. In Dioscorea and 

 Sedura all the bundles are typically collateral. The same holds good for the Ophrydesc, 

 with the limitation that the vessels are only very sparingly developed. Each bundle is 

 surrounded by a separate endodermis. 



4. In the Filices in the widest sense, the Marsiliacese, and the Equiseta, with a 

 few exceptions to be mentioned below, the axial cylindrical bundle of the roots does 

 not deviate in its differentiation from the types hitherto regarded ^ Its xylem is in 

 the great majority of cases, with the exception of the Marattiaceffi, diametrally diarch, 

 beginning externally on each side with some narrow, fibrously-thickened tracheides 

 lying side by side, which are succeeded in the centripetal direction by one or a few 

 rows of wider, often large scalariform tracheides, of the structure usual in terns ; 

 (true vessels only occur in Athyrium Filix femina "). Cf. Fig. 1 69. In Botrychium, the 



» Compare NageU und Leitgeb, van Tieghem, Russow, H.cc. ■' Compare p. 165. 



