1917] Butters,— Studies in Ferns — Athyrium 175 
subtending vein and lies upon the sporangia. In the genus Athyriwm 
such sori seem always to be in close contact with the subtending vein 
throughout their entire length, and never, as in some species of 
Asplenium, to curve away from it at the ends. Asplenioid sori vary , 
greatly in length, but are otherwise quite uniform in character. The 
indusium is raised and pushed back by the developing sporangia, and 
at maturity is often completely reflexed. 
The sorus characteristic of the great tropical genus Diplazium and 
found also in some species of Asplenium and Athyrium, consists of two 
i age 
Fig. 3. Fig. 4. 
Fig. 3. Asplenioid sori. A, B, Asplenium platyneuron (L.) Oakes. A, pinna, X 2}; B, 
young sorus, X 10, the sporangia indicated as if seen through the transparent indusium. C, 
sorus from the distal portion of the pinnule of Athyrium Filiz-femina (L.) Roth., l-1, line meas- 
- uring the length of the indusium, h-h, line measuring its height. 
Fig. 4. Diplazioid sori. A, Diplazium acrostichoides (Sw.) Butters, X 12}; B, Athyrium 
asplenioides (Michx.) Desv., X 124; C, portion of the frond of Diplazium plantaginifolium (L.) 
Urban, natural size. 
asplenioid sori standing back to back along the same vein (see fig. 4). 
Such sori may vary not only in their absolute length (in some species 
of Diplazium reaching over 2 cm.) but also‘in the relative length of 
the component parts. In general, the anterior sorus is longer than 
that on the posterior side of the vein. In typical Diplaziwm the 
indusia wither at maturity. 
The characteristic athyrioid sorus may be described as an asple- 
nioid sorus which at the distal end crosses the subtendin g vein (see 
fig. 5). Athyrioid sori display great diversity of form. At the distal 
