SUMMARY 425 
evolutionary priority in face of strong comparative evidence to the contrary. 
Accordingly the existence of the complex and definite cone of Cheirostrobus 
as the earliest of the Sphenophyllales cannot be held as invalidating the 
conclusion above stated. 
In all these sporangiophoric Pteridophytes the axis is the dominant 
part of the shoot, and takes the lead in its development. This is shown 
anatomically by the typically protostelic structure of Sphenophyllum, while 
the foliar strands insert themselves marginally upon it. In the Psilotaceae 
the type of stem-structure is the same, but the xylem becomes hollow 
and medullated in the upper region, and in Zmesipteris it may break 
up into separate strands. Even in the apparently divergent case of the 
Equisetales it has been shown that the vascular system is referable to 
a primitive type of monostele, though greatly reduced in accordance with 
a semi-aquatic habit. Notwithstanding such changes the structure of the 
axis in all these forms indicates origin from a type in which the axis is 
predominant over the appendages, the vascular’ supply of these being 
inserted with the minimum of disturbance upon the cauline stele (Clado- 
siphonic type of Jeffrey). This confirms the theory of a strobiloid origin, 
with pre-existent axis and subsidiary appendages. 
The differences in number of the sporangia on the individual 
sporangiophore call for remark. In the Sphenophylleae they have been 
observed to vary from six in Sphenophyllum majus to one only in 
S. Dawsoni, while Pstlotum and Tmesipteris take a middle position with 
three and two respectively. In the Equisetales the number in the 
fossil form appears to be commonly four, but in recent species of Eguisetum 
the number may be much larger and variable! In the Sphenophyllales 
the variations present some points of interest: the number six occurs 
only occasionally in the one species named, while four is the usual 
number in that species, which it shares with Cheirostvobus. But in the 
species of Sphenophyllum with compact strobili the number may be two, 
or only one. These low numbers go along with a larger number of the 
sporangiophores, which may be twice (5. Dawsoni), or three times 
(S. Rémert) the number of the subtending bracts. In the former species 
the frequent juxtaposition of the stalks, and the insertion of the vascular 
supply of the stalks upon the strands supplying the bracts, suggests that 
fission has been operative, as in the chorisis of stamens: and it seems 
probable from the facts that with an increase of number of the sporangio- 
phores, however brought about, there has gone a decrease in the number 
of the sporangia which each bears. Accordingly S. Dazsonz and S. Romeri 
may be held to bear sporangiophores of a type reduced from the original: 
and a central type of sporangiophore would appear to be one with about 
four sporangia. 
The typical position which such a  sporangiophore holds in the 
Sphenophyllales is one of attachment in a median position to the upper 
1In Calamites paleaceus the pendulous sporangiophore bears a solitary sporangium. 
