4 2 4 SPORANGIOPHORIC PTERIDOPHYTES 



Psilotaceae, and apparently axillary in the Sphenophylleae. But the analogy 

 of Equisetum is interesting in this relation, for there, though the normal 

 branching is monopodial, a terminal branching of the strobilus sometimes 

 occurs. Thus the points of resemblance appear greatly to outweigh the 

 differences, and the Psilotaceae and Sphenophylleae may well be grouped 

 together as representing one phylum the Sphenophyllales. 



Examining the plants thus designated from the point of view of a 

 strobiloid theory, the Psilotaceae show in the most pronounced way the 

 " Selago" condition, while about the limits of the fertile zone in Tmesipteris 

 imperfectly formed sporangiophores are often seen, which bear a similar 

 interpretation to the abortive sporangia in Lycopodium. The matter is 

 complicated here, it is true, by the marked difference between the simple 

 sterile leaf and the forked sporophyll : it has been shown, however, that 

 in the first stages of the individual development these parts are indis- 

 tinguishable. Though in Sphenophyllum the strobilus is definitely marked 

 as a rule from the vegetative region, it is important to note that 

 Sphenophyllum majus, which most nearly resembles the Psilotaceae in the 

 form of its appendages, has also an indefinite strobilus, with continuation 

 again upwards into a vegetative state. Thus in both families the shoot 

 shows examples of imperfect differentiation. This goes along with a 

 development of the sporophyll, both in Tmesipteris and in Sph. majus, 

 as an effective organ of assimilation, which is a further mark of a low 

 differentiation. These facts may be held as justifying for the Sphenophyllales 

 a line of argument similar to that for the Lycopodiales : that a definite 

 strobilus has been the result of differentiation in a shoot in which the 

 vegetative and reproductive functions were not originally separate. It is 

 true that the case is not so clear for the Sphenophyllales as for the 

 Lycopodiales : this is consequent on the number of the known species 

 and of individuals available for comparison being much less, and the 

 knowledge of the fossils more limited. It will perhaps be objected that 

 the earliest known cone of this series, Cheirostrobus, was perfectly definite 

 and highly specialised, while the earliest indication of a less specialised 

 type is in Sph. majus. But the fact that so often the earliest known 

 fossils of any phylum are very complex does not prove that such types 

 were earliest in evolution. Thus Cheirostrobus among the Sphenophyllales 

 and Pseudobornia among the Equisetales, though respectively the oldest 

 representatives known, are both extreme forms, as compared with the other 

 members of the phyla to which they respectively belong. In estimating such 

 facts we should reflect that at the present day primitive and recent forms 

 grow side by side, and both or either might be preserved as fossils ; also 

 that the chances of this happening depend upon many varied factors, of 

 opportunity, texture, habitat, etc. The chances of discovery at the present 

 day are equally varied. When these points are fully considered it will 

 be clear that stratigraphical position of those isolated fossils which happen 

 to have been discovered and described should not suffice to prove an 



