STRUCTURE OF RADIAL BUNDLES. ^6^ 



branched, and show a definite orientation of the parts of the bundle in the successive 

 bifurcations. The structure in question might therefore be regarded as characteristic 

 of dichotomous roots, if it were not that those of Ophioglossum are always entirely 

 unbranched \ for there is no basis of fact for van Tieghem's supposition, according 

 to which this unbranched root would be the favoured branch of a root which has 

 already undergone bifurcation while still inside the cortex of the stem which pro- 

 duces it, its other branch not coming to development. 



Among the dichotomous rhizophores^ and roots of the Selaginellae, in the first place, 

 the rhizophores of S. Kraussiana are distinguished by cylindrical vascular bundles, in 

 which the middle of the central and centrifugally developing xylem is occupied by the 

 narrow primitive tracheides, while the periphery is formed of wider scalariform tra- 

 cheides. The phloem completely surrounds the xylem as a many-layered small-celled 

 zone ; histologically it still requires more exact investigation ^ To form the bundles of 

 the first pair of roots, the bundle of the rhizophore is uniformly severed into two halves, 

 in which a group of narrow primitive tracheides occupies one edge of the xylem, while 

 the development of the elements proceeds from this point towards the other broader 

 side. The xylem is thus monarch, similar to the usual collateral bundles, from which 

 those in question are distinguished by the fact that the phloem completely surrounds the 

 whole xylem. 



The structure last described belongs to all the investigated roots of Selaginella, and to 

 the rhizophores of S. JNIartensii. These bundles also divide at the dichotomies in such 

 a manner that the plane of division passes through the primitive group and the edge of 

 the xylem lying diametrically opposite to it. In primary axes arising from the stem, the 

 orientation of the bundles with a unilateral group of primitive tracheides is such that 

 that group faces the base of the stem. In the dichotomous branches it always lies on the 

 inner side, turned towards the other branch of the pair. At every bifurcation therefore 

 each bundle proceeding from the division of the main-bundle undergoes a torsion of 90°, 

 and indeed this takes place gradually inside the main axis, in such a way that the two 

 bundles run on side by side from the point where they separate, which is above the point 

 of bifurcation of the root, as far down as the latter. Only in the first dichotomous 

 branches of the rhizophore of S. Kraussiana does the same orientation come about 

 without torsion. 



The feeble bundles in the roots of Isoetes* show in the general structure of their uni- 

 laterally monarch xylem, and in its orientation in the dichotomous branches, the same 

 behaviour as those of Selaginella. As regards the elementary composition of this part 

 they are distinguished by the fact that it consists only of a few rows of annular and 

 reticulated tracheides, without scalariform vessels. So far as investigations extend, the 

 phloem is feebly developed, and confined to the side remote from the primitive tra- 

 cheides ; as seen in cross-section it has the form of a narrow crescent-shaped band, wh ile 

 its histological structure is indistinct. The position of the bundle in the root is 

 from the first slightly eccentric, and in the dichotomous branches always approaches the 

 other branch of the pair. The eccentricity, which is caused by a mainly one-sided 

 extension of the cortex and of its large air-cavities, increases with the thickness of the 

 roots. According to Mettenius' short statement ^, the structure of the xylem and the 

 eccentric position of the bundle in the roots of Phylloglossiim are similar to those in Isoetes. 

 The bundle approaches that side of the always unbranched root which is basiscopic with 

 reference to the stem. 



' Compare Holle, Botan. Zeitg. 1875. HoUe's observations are not in agreement with van Tieg- 

 hem's statement that the roots of Botrychium, with typically radial bundles, are dichotomous (p. 315). 

 2 Nageli und Leitgeb, /. c. p. 124. s j-^f Treub, /. c] 



* Hofmeister, Beitrage z. Kenntniss d. Gefasskryptogamen, I. — Nageli und Leitgeb, I.e. p. 131. 



* Botan. Zeitg. 1867, p. 99. 



