386 Watson. — On Mesostrobus , a New Genus of 
This arrangement has certain obvious disadvantages : — 
1. The attachment-area of the sporangium is so limited that provision 
of sufficient food material to the developing spores would be a difficulty. 
2. It is weak mechanically. 
Both these disadvantages will be removed if we increase sufficiently the 
attachment area of the sporangium. This increase can only be made by 
extending the attachment of the sporangium down the horizontal portion of 
the sporophyll towards the axis. 
In this way we reach a condition which is preserved for us in Meso- 
strobus ; here we have considerable radial extension of the sporophyll 
combined with attachment of the sporangium to it only along its distal 
half. It is noteworthy that the Mesostrobus sporophyll strongly recalls that 
of Bothrodendro 7 t mundum. 
Further continuance of this process leads directly to the ordinary 
Lepidostrobus (cp. Text-fig. 2). 
Text-fig. 2. The rise of the Lepidostrobus condition. A = Bothrodendron mundum ; 
B = Hypothetical ancestry corresponding to Spencerites ; C = Mesostrobus ; D — Lepidostrobus . 
Spencerites does not appear to me at all the sort of thing one would 
expect, if it were a cone retaining very archaic features. 
Certain specimens, at any rate, have their sporophylls arranged very 
regularly in alternating verticils (cp. Text-fig. 3). Now the vast majority 
of Lycopods have not a trace of verticillate arrangement, and where such an 
arrangement does occur it is usually in the case of a cone. 
In a cone it is easily seen that a verticillate arrangement with 
alternating verticils is that which will secure adequate protection for the 
sporangia with the least possible area of lamina. 
The importance of this saving is well illustrated by the fact that in the 
cone Calamostachys Binneyana , whilst the sporangiophores are superposed, 
the bracts are in alternating verticils ; the whole leading to a very curious 
arrangement of leaf-traces, which is not yet understood. 
A far more striking case, however, is that of Sphenophyllum Dctwsoni , 
where, although the facts are not absolutely conclusive, it seems that the 
bracts alternate, although such an arrangement is hard to correlate with the 
vertical course of the protoxylems, and with the fact that the superposition 
