595 
Centripetal Xylem in Equisetum , 
Equisetales and the Sphenophyllales. The latter possess a stolid stele 
of centripetal wood. Catamites pettycurensis has a considerable amount of 
centripetal xylem ; and even the axial bundles of Equisetum show clear 
centripetal wood in some cases. 
Cross-sections of the cones of E. hyemale and E. fiuviatile show 
bundles as described above (p. 594) with this addition : upon the inner 
side of the lacuna are often found, adjacent to the canal, or separated 
therefrom by a few parenchyma cells, from one to four or five tracheides. 
Radial sections (Fig. 6 of E. hyemale) show these to be metaxylem, 
scalariform or reticulate elements, situated on the inner side of the broken- 
down spiral and ringed elements of the protoxylem. With its considerable 
body of metaxylem upon the external side of the lacuna, the reproductive 
branch possesses fibro-vascular bundles of the type that has been called 
4 mesarch \ l 
The reproductive axis of Equisetum gives accordingly some evidence 
of the derivation of the genus from Calamitean forms similar to C. petty- 
curensis, Still stronger evidence is provided, however, in the foliar traces 
emanating from this region. The strobilus consists of a continuation of 
the stem, with much shortened internodes bearing, at the nodes, whorls 
of modified leaves, the sporophylls. The bundles of the sporophylls were 
studied in transverse and longitudinal sections in the following species : — 
E. maximum, E, hyemale, E, fiuviatile, E. arvense, E. palustre, E. sylvati- 
cum, and E, scirpoides. In all save the last two strongly-marked mesarch 
strands appear. In external appearance the sporophylls vary but slightly. 
Their fibro-vascular system, however, shows considerable difference in 
development, due to the great range of habitat found in the genus — from 
xerophily in E. scirpoides and E. arvense to hydrophily in E. fiuviatile. 
But even in the latter the cone is not weak in vascular elements. In nearly 
all species a lateral fusion of sporophyll-peduncles occasionally occurs, and 
the union of their flattened distal ends is often found. Figs. 7 and 8 
show cross-sections of sporophylls of E. maximum and E. fiuviatile 
respectively, the former near the middle of the stalk, and the latter through 
the trace as it leaves the cortex of the strobilus. Both show well-defined 
lacunae in the centre of the bundle. Above and below, and to some 
extent also upon the sides, are tracheides. The longitudinal sections 
were cut in the dorsiventral plane of the sporophyll. Looking now at 
such a section (Fig. 9) from the fertile leaf of E, fiuviatile, the structure 
seen in transverse section is explained. The elements upon the left are 
ventral, corresponding to the upper group in the cross-section ; those upon 
the right are dorsal tracheides. Between, is a partially open space, the 
1 The term * mesarch ’ was first applied to such structure by Count Solms-Laubach, This type 
of arrangement in bundles has since been used to a considerable extent in the determination of 
relationships among the groups of Vascular Cryptogams. 
S S 
