178 Sinnott . — The Evolution of the Filicinean Leaf trace. 
The only remaining members of the Gradatae to be mentioned are the 
Cyatheaceae. These are tree-ferns with large stems, and display consider- 
able complexity in their stelar system and in their leaf-bundles. Alsophila 
pruinata , as described by Karsten (18), shows the simplest conditions in the 
family — a siphonostele with the leaf-trace departing as a single strand. 
A study of the development of young plants of Alsophila excelsa by Gwynne- 
Vaughan (15) makes it clear that the advanced adult state has come through 
the protostelic and siphonostelic stages, as in all ferns, and that the leaf- 
trace was originally a single bundle. This latter conclusion was borne out 
by the writer’s examination of some young plants of Cyathea Macarthuri , 
in which the base of the leaf-trace is a single triarch strand in the shape of 
a triangular arch and much resembles the leaf-bundle of Ettgleichenia. This 
trace soon becomes separated into two diarch bundles of the Onoclea type, 
each of which later divides into two again (Text-fig. 9 ). By subsequent 
division the complex adult leaf-trace is formed. 
In the position of thesorus and the structure of the sporangium, as well 
as in the above-mentioned anatomical resemblance, the Cyatheaceae show con- 
siderable similarity with the Gleicheniaceae, among the Simplices, and have 
probably arisen from somewhere in their vicinity. The family constitutes an 
independent line of advance, and seems to have led to no higher forms. 
The Mixtae are usually all included in the Polypodiaceae. This great 
family comprises the vast majority of living ferns, and is usually divided 
into two groups : those with marginal sori, or the Pterideae, and those with 
superficial sori. 
The Pterideae seem to have had a somewhat different origin from the 
rest of the Mixtae. The recent researches of Professor Bower on the genus 
Plagiogyria ( 8 ) prove that though this fern possesses sori which are marginal 
and clearly of the mixed type, they show no signs of having passed through 
a basipetal or gradate condition. In its habit, in its anatomy, and in the 
structure of its sporangium, this genus presents close resemblances to several 
families of the Simplices. It probably is a primitive form and near the 
ancestral type of the Pterideae, which thus seem to have developed a mixed 
sorus directly from a simple one. 
Plagiogyria possesses a simple mesarch dictyostele from which the 
leaf-trace departs as a single, concentric, arched strand in the form of 
a rather broad inverted V. In this are three protoxylem groups, one 
median and two lateral. The median one, and from the figures apparently 
the other two as well, is at first distinctly mesarch (Text-fig. 6 ), but as the 
trace passes through the cortex it gradually becomes endarch. The median 
group soon divides into two, and this arch, very similar to that of the Denn- 
staedtineae, continues through the leaf. The early condition of the trace, 
therefore, shows a very close approach to our primitive bundle, being 
triangular in shape with three mesarch protoxylem groups. 
