Anatomy of Acrostichum aureum. 187 
function somewhat as the “ prop ’’-roots of other mangroves. As a 
mere speculation, it seems to me even more probable that the 
Stigmarian axes of the ancient Lycopods were prop-roots, for they 
were known in many cases to “ strike sharply down at an angle of 
50° or 60° and only begin to take a horizontal course at some 
distance from the main trunk. 1 ” Their anatomy lends support to 
the view by analogy with that of the prop-roots of flowering plants' 
When we consider the part that these members have to play, 
it is not surprising that the anatomical structure is intermediate 
between that of a root and a shoot, for the mechanical requirements 
of both must be met. On the girder system a medullated axis is 
recognized as most suitable to support a weight, while a central 
solid core best withstands the strains resulting from the forces to 
which a root is subjected. 
Prop-roots in tidal swamps must be liable to stresses from the 
shifting nature of the surrounding medium, and in accordance, on 
the one hand, with the amount of weight which the structure is 
called upon to bear, and, on the other hand, with the amount of 
strain induced, a preponderance of shoot or root-characters may be 
expected. 
In this connection it may perhaps be permitted to hazard 
an almost unsupported suggestion. 
The root characters common to our fern, to the mangroves, 
and to fossil plants particularly of Lycopsidan affinity, have been 
sufficiently insisted upon ; also the fact that these characters 
obtain largely in the living members of the latter phylum. 
It seems then that an imperfect differentiation of root and 
shoot is characteristic of the Lycopsids and may be correlated 
with the comparative insignificance of their leaves. Lycopsida 
undoubtedly bulked largely in the coal measure vegetation, and 
evidence seems to be stronger as to the petrifaction of these plants 
in situ than in the case of members of other groups. We may per¬ 
haps take this as far as it goes as indicating suitability to swamp 
conditions. 
The lacunar character of the plants, the imperfect differentia¬ 
tion of root and shoot, and the small leaves are all consistent with, 
and perhaps partly due to a swamp life, with its plentiful water 
supply and peculiar mechanical conditions. It may be that their 
descendants were never quite able to throw off the shackles of 
their ancestry, and to this day retain many of the characteristics 
of their forbears, together with a moisture-loving habit. 
1 Scott, “ Studies in Fossil Botany,” p. 220. 
