36 Tans ley and Chick. — Notes on the 
is developed in the sporophyte) instead of merely leading 
up to the base of the sporogonium. The lineal descendant 
of such a primitive hydrom-stele would then perhaps be seen 
in the central metaxylem of, for instance, Sphenophyllum , 
Ckeirostrobus , the Lepidodendra with solid steles, the mono- 
stelic Selagineltas , and (modified in various ways) in Psitotum , 
Lycopodiu7n , &c. Added to this would be the bases of the 
leaf-traces, represented by the peripheral protoxylem-strands 
(separate, or as in Lepidodendron laterally confluent) and 
only evolved after the primitive sporophyte had thrown out 
leaves requiring a vascular supply connected with the main 
channel of the stem. The fact that they appear before the 
central xylem in the development of the individual stem 
would be merely in relation to the need for the early estab- 
lishment of conducting channels to the leaves, a need which 
is universal in leafy vascular plants. 
On the other hand we might suppose that the formation 
of leaf-structures requiring a vascular supply preceded the 
formation of a regular stele supplying the fruit-body, in which 
case the leaf-traces, represented in the first place by the 
protoxylem-strands, would be phylogenetically prior, and the 
central metaxylem would be a later formation, developed in 
the larger forms to furnish additional conducting channels 
to supplement the protoxylems in supplying the needs of 
the higher foliage-leaves and the sporophylls. 
And if, setting aside the antithetic theory, we imagine the 
evolution of the sporophyte from the alternate generation 
of a homogenetic thalloid form, the elements of the problem 
are not essentially changed. It is still, as it appears to us, 
a question of the priority in evolution of leaf-structures on 
the one hand, or of a differentiated conducting strand on the 
other. In the latter case the metaxylem of the protostelic 
Pteridophyte may represent a primitive water-conducting 
strand in the centre of a thallus, such as we find in Paltavi- 
cinia ; in the former, merely a later-formed supplement to the 
connected system of leaf-traces. 
