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 " Selago " section of Lycopodium^ with its successive, but little 

 differentiated, sterile and fertile zones. But not uncommonly the fertile 

 zones of Tmesipteris 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 : x 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 Psilotum the main features resemble those in Tmesipteris, 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. flaccidnm, 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 Tmesipteris, but are more profusely 

 bifurcate : they are covered with rhi/,oids, and show mycorrhiza. They 

 produce gemmae, which freely propagate the plant vegetatively. The 

 aerial shoots also bifurcate much more freely than in Tmesipteris, 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 Tmesipteris, though very small : each bears a short-stalked sporangio- 

 phore, which supports three synangial sporangia. Here as in Tmesipteris 



1 Many of the details here embodied arc taken from Thomas, Proc. Roy. Soc., vol. l\i\., 

 P- 343- 



