The Grouping of Vascular Plants 83 
In the Sphenopsida we find leaves which are also probably 
cladodified axes but they show a definite trend towards a repeated 
dichotomy of the veins and in many cases a palmate type of seg¬ 
mentation of the leaf. For such simply elaborated primitive leaves 
the term “Meiophylls” is suggested. 
In the Meiophyll the branching is always a dichotomy in one 
plane so that the leaf is strap-shaped or flabelliform. In the inter¬ 
esting cases of vegetative branches of Sphenophyllum bearing both 
laciniate and entire Meiophylls, the entire are found to be lower 
down on the axis. Apparently the earliest record of the Meiophyll is 
to be seen in Hyenia sphenophylloides Nathorst (Middle Devonian) 1 
where the appendages are either simple or dichotomously branched. 
Such appendages may have given rise equally to the segmented 
and the non-segmented cladodified leaves of the later species of 
Sphenophyllum. The same range occurs in the pinnae of the early 
Fern Meriphyte such as Sphenopteris and it is obvious that the 
Meiophyll is homologous with such pinnae. Though certain Fern 
pinnae show this type of venation the complete leaf involves 
greater complexity. 
Pseudohornia exhibits, so far as the writer knows, the most 
elaborated leaf of the Meiophyll type. It would be interesting to 
secure structural material of this variant. 
In the “cone scale” of the Sphenopsid Cheirostrobus we find both 
soromata (sporangiophores) and a segmented bract-like structure. 
The dorsiventral branching of the so-called ‘‘cone scale” trace in 
the cortex of the axis is not to be regarded as a branching of the 
leaf trace, but the branching of the common axis trace which on 
dichotomy in the dorsiventral plane gives rise on the ventral surface 
to sporangiophores (axial structures) and on the dorsal surface to 
the Meiophyll which proceeds normally to branch in a palmate 
manner. Thus the very antiquity and hence primitiveness of this 
remarkable cone, so magnificently described by Scott, really is at 
the root of its complexity. 
There is yet a third type of leaf among the Vascular Cryptogams, 
namely the complex meriphytic leaf of the Ferns. This type has 
been fully discussed by Lignier and others whose theories were 
critically examined and accepted by Tansley 2 in 1908. The extra¬ 
ordinary resemblance between what we regard as the leaf of 
1 Arber, Devonian Floras. Camb. Univ. Press, 1920. Fig. 25. 
2 Tansley, Lectures on the Evolution of the Filicinean Vascular System, 1908. 
New Phytologist, Reprint, p. 2. 
