596 Eames. — On the Occurrence of 
lacuna of the cross-sections. The origin of the latter is the same as that 
of the carinal canal of the stem. The elements outside the disarranged 
spiral and annular tracheides of the lacuna are closely ringed or scalariform. 
This is, then, a mesarch bundle. The dorsal elements are of course centri- 
fugal, the ventral, centripetal. It might, perhaps, be objected that this 
arrangement of elements in the sporophyll-bundle is due to the concentric 
arrangement of the sporangia. But this is not the case. The strand 
continues in the general form shown in Fig. 8 until it approaches the base 
of its flattened end. There occurs a division into a number of bundles 
corresponding to the number of sporangia. Fig. to shows this con- 
dition in E. maximum. In the lower portion the undivided bundle is 
seen. Its resemblance to that of E. fluviatile is close. There is a central 
mass of distorted protoxylem bounded by unbroken tracheides. As the 
bundle passes outward division is made into several parts, only two of 
which, of course, lie in the plane of section. The separation is made from 
the very centre, a portion of the protoxylem passing to each branch-strand. 
The latter immediately forms metaxylem upon its inner face. At the very 
top of the figure this can be seen. And this mesarch structure in each 
of these small bundles can often be traced clear to the base of the spor- 
angium. The arrangement of the sporangia upon the tip of the sporophyll 
cannot, then, control the form of the main bundle, causing it to simulate 
mesarch structure. The protoxylem of the two species discussed above 
consists chiefly of loose-ringed tracheides. Two other species are inter- 
esting in comparison: E. arvense (Fig. 11) shows portions of a spiral 
tracheide in the protoxylem ; the central elements of E. palustre (Fig. 12) 
are spiral throughout. 
The sporophyll fibro-vascular bundles appear not only to be mesarch 
but concentric, a condition which may also be regarded as a relic of 
ancestral structure. 
Jeffrey 1 has shown that ‘the Equisetaceous strobilus perpetuates both 
the non-alternating strands and the complete absence of foliar-gaps of 
the oldest Calamitean forms ’. These figures make it probable that it 
also retains, so far as the sporophylls are concerned, the bundle-structure 
of ancestors contemporary with C. petty cur ensis. 
In the trace of the vegetative leaf the mesarch condition is also found. 
Investigations were made by means of radial sections of complete nodes 
with leaves attached, by cross-sections of the stem just above the origin of 
the traces, and by tangential sections at the outer limit of the supranodal 
wood. Well-marked centripetal wood was found in E. maximum , E. kye- 
male , E. hyemale var. robustum , E. arvense , and E. scirpoides. In E. sylvan 
ticum and E. fluviatile , species of shaded and aquatic situations, the 
1 Jeffrey, E. C. : Are there foliar Gaps in the Lycopsida? Bot. Gazette, vol. xlvi, p. 254. 
1908. 
