174 Sinnott. — -The Evolution of the Filicinean Leaf -trace. 
two sub-genera, this type of trace is not seriously modified. In Mertensia „ 
however, more complex conditions appear, due to the approach towards 
siphonostely. Platyzoma represents the extreme of modification. 
The last family included under the Simplices is the Matonineae, 
represented by the single genus Matonia , which includes two species, 
M. sarmentosa and M. pectinata . The former is very simple in structure, 
its central cylinder, as observed by Compton (12), being an endarch 
amphiphloic siphonostele, surrounding one large medullary bundle. The 
leaf-trace departs from the stele as a single concentric strand in the shape 
of a triangular arch, and with three endarch protoxylems, which persist 
undivided throughout the petiole. M. pectinata, on the other hand, is much 
more complex, possessing a central cylinder which contains often as many 
as three concentric siphonosteles. The leaf-trace goes off as a single wide 
arch, which contains at its very base five endarch protoxylem groups. This 
number becomes greatly increased in the petiole, where the bundle is much 
larger and more complex. Young plants of this species show but a single 
siphonostele and a much smaller leaf-trace. This would apparently be 
triarch, if typical protoxylem, which is very late in appearing, could be 
made out. 
M. sarmentosa seems to be clearly the more primitive of the two 
species. In its fronds as well as in its vascular anatomy it resembles young 
plants of M . pectinata, and it is also very much nearer to the Gleicheniaceae, 
the family which the Matonineae most closely approach in sporangial and 
soral characters, and to which they are doubtless nearly related by 
descent. 
In the Gradatae, that group characterized by Bower as possessing sori 
in which there is a definite succession in time and space in the development 
of the sporangia, the first family to be considered is the Hymenophyllaceae, 
the anatomy of which has been carefully investigated by Boodle (3). These 
small, filmy-leaved ferns exhibit considerable diversity of structure, the 
central cylinder ranging from a collateral or nearly collateral bundle in 
the very simplest forms, to a much larger protostelic mass in which the 
first-formed elements of the xylem may be indefinitely scattered, may be 
massed with parenchyma in a pith-like island in the centre, or may be 
distributed in an exarch position along the periphery. The leaf-trace is 
a single, usually collateral, strand, which in the simpler forms is much like 
the central cylinder and possesses one protoxylem group. In the slightly 
more complicated species, such as Hynienophyllnm dilatatum v. Forste- 
riantim and Trichomanes reniforme , where the stele consists of a ring of 
metaxylem surrounding a group of parenchyma and protoxylem, the 
leaf-trace at its base is very similar to the central cylinder, but higher up 
the xylem changes from a ring to an arch, and the protoxylem splits into 
two endarch groups. In certain species of Trichomanes, such as T. Prieurii 
