593 
Centripetal Xylem in Equisetum. 
in its order having in the Filicales and Lycopodiales no observable effect 
upon the sculpture of the cell-wall, most stress in deciding the morpho- 
logical order of development must be laid upon the latter. Judged by this 
standard the flanking xylem-groups of Equisetum are centrifugal in their 
development. 
It is consequently evident that all parts of the stem-bundle develop in 
the same direction. Had there been no apparent proof that the respective 
portions of this much-disputed bundle were of varying differentiation, doubt 
would not, in all probability, have been cast upon its simple nature. 
Gwynne-Vaughan, 1 in his statement that only the protoxylem-strand is 
concerned in the formation of the vegetative leaf-trace, has brought forward 
evidence for the complex nature of the internodal bundle. Lignier (see 
above-mentioned article, p. 114, note) is of the same opinion in regard to 
the origin of the leaf-trace. On the other hand, Campbell 2 and De Bary 3 
seem to understand the leaf-trace as connected with the entire internodal 
bundle. Dr. Scott’s position on this point is clear. In his ‘ Studies in Fossil 
Botany ’ (1908), he says (p. 25) : 1 A single vascular bundle enters the stem 
from each leaf and passes straight down through one internode only. 
At the node below it forks.’ The bundle in its entirety is clearly indicated. 
Fig. 3 appears to settle this point at once. The photograph is of a 
portion of a radial section through the node of E. hyemale var, robustum. 
In this species the metaxylem is very well developed, and its share in the 
formation of the leaf-trace can be easily seen. In other species it is less 
easy to observe. Differences of opinion on this point are not hard to 
explain. Even though the tracheides of the metaxylem are related to those 
of the protoxylem at the point of origin of the leaf-trace, they are still 
situated upon its flanks and a radial section through the very centre, 
showing the departure of the protoxylem, does not usually also show well 
the relation of the metaxylem to the trace. And a section passing through 
the metaxylem might readily be cast aside as not strictly radial ; especially 
would this be likely to occur where the metaxylem is small in amount, and 
the section contains none of the protoxylem. The metaxylem unques- 
tionably takes part in the formation of the leaf-trace. In Fig. 4, which 
illustrates another feature of the leaf-trace, further evidence for the 
centrifugal development of the metaxylem is presented. The tracheides 
upon the right of the protoxylem are the direct continuation of the elements 
given to the leaf-trace by a lateral metaxylem-strand in Fig. 3. Their 
order of development is definitely outward. 
The unit nature of the cauline internodal bundle becomes further 
1 Gwynne-Vaughan, D. T. : Remarks upon the Nature of the Stele in Equisetum. Ann. Bot., 
vol. xv, p. 775 ; 1901. 
3 Campbell, D. H. : Mosses and Ferns, p. 462 ; 1905. 
3 De Bary, A. : Comparative Anatomy of the Phanerogams and Ferns (trans.), p. 279 ; 1884. 
