GENERAL MORPHOLOGY 411 
among the bifid sporophylls: not uncommonly there is a reversion from 
the strobilus back to the ordinary vegetative state. In fact, as regards 
relation of foliage leaves and sporophylls, the condition is the same as 
that in the “ Sedago” section of Lycopodium, with its successive, but little 
differentiated, sterile and fertile zones. But not uncommonly the fertile 
zones of Zmesipieris show differences from the normal as regards the details 
of the spore-bearing members at the limits, or about the middle of the 
fertile zones:1 about the upper and lower limits, but especially at the upper, 
variations of reduction from the normal, both of sporophylls and of 
synangia may be found: these may appear in the abortion of either 
loculus, or of both of them (Fig. 228 i. ii. iii.) : or the two loculi may be 
imperfectly formed, the septum being incomplete between them, and the 
synangium is then replaced by a single loculus (Fig. 228 lower row). It 
would ‘appear that these reductions are to be correlated with deficient 
nutrition at the limits of the fertile zone. Conversely, about the middle of 
a fertile zone, where presumably the nutrition at initiation of the parts is 
most efficient, certain sporophylls may be developed beyond the normal 
limits: in the simpler cases an additional loculus may appear in the 
synangium (Fig. 228); but in well-developed plants Thomas has found 
that not infrequently the sporophylls may show a repeated dichotomy, and 
two or even three normally shaped synangia, or sporangiophores, may be 
produced, one at each fork of the sporophyll. He has also described how 
the sporangiophore is not always sessile as it is normally, but may be 
raised up on a longer or shorter stalk ; also that it may at times be replaced 
by a leaf-lobe of outline like those which are normal. The theoretical 
bearings of these several variations, which do not appear to be uncommon 
where the plant flourishes well, will be discussed later. 
In Psdlotum the main features resemble those in Zmesipiteris, but with 
differences of detail. The genus consists of two well-marked species, 
P. triquetrum, which is upright and shrubby, with a radially constructed 
shoot, and P. flacidum, which is weak and pendulous, with a bilaterally 
flattened shoot, bearing the appendages on its margins. The underground 
rhizomes are rootless and leafless, as in ZmeszPteris, but are more profusely 
bifurcate: they are covered with rhizoids, and show mycorrhiza. They 
produce gemmae, which freely propagate the plant vegetatively. The 
aerial shoots also bifurcate much more freely than in Zmesipteris, in 
planes successively at right angles (Fig. 229). On these the minute vege- 
tative leaves are disposed, but with no constant or definite arrangement : 
they appear as small subulate processes, arising from the projecting angles 
of the green axis, and are commonly without vascular tissue. In the upper 
regions of strong shoots they are replaced by sporophylls which are bifurcate 
as in Zmesipteris, though very small: each bears a short-stalked sporangio- 
phore, which supports three synangial sporangia. Here as in Zmesipieris 
2Many of the details here embodied are taken from Thomas, Proc. Roy. Soc., vol. Ixix., 
P. 343. 
