154* O. sensibilis L. Sensitive Fern. Sterile fronds 6 inches 
to 3 feet, scattered along a creeping rootstock. Laminre triangular- 
ovate, deeply sinuate-pinnatifid, reticulate veined. Fertile fronds 
bipinnate, much contracted, their revolute segments berry-like. 
Sterile fronds throughout the summer ; fertile fronds middle of 
September. Very sensitive to frost. Abundant in damp soil. 
United States east of the Mississippi, and Japan. 
Illustrated by Meehan, Series ii, Vol. 1. 
a. Obtusilobata is an occasional form in which the normally 
sterile frond has its segments revolute, enclosing a few usually 
abortive sori. Fronds of this nature are usually quite small, and 
are to be expected in situations where the fern has suffered an in- 
jury, as by cropping early in the season, or where growing in very 
dry situations. August. 
155. O. Struthiopteris (L.) Hoffm. Ostrich Fern. Sterile 
fronds 1-10 feet, in a crown ; laminre pinnate, the pinnae recurved 
and deeply pinnatifid ; pinnae numerous, becoming gradually small- 
er towards the channelled stipe. Fertile fronds 6-18 inches, con- 
tracted, simply pinnate ; margins of the pinnae revolute into a 
necklace form containing the sori, Along streams and in moist 
places ; most common in western sections. 
This fern when growing in dry situations, especially if the ster- 
ile fronds have suffered injury, sometimes produces fronds anala- 
gous to those of O. sensibilis obtusilobata. Sterile fronds through- 
out the summer ; the fertile late in August. Northern Europe and 
Asia. 
WOODSIA R. Br. 
Sori roundish ; indusium fixed beneath and enclosing the sori, soon 
bursting and developing a hairy fringe, or in case of the last 
species, splitting into irregular lobes. Four species in New 
England. 
156. W. Ilvensis (L.) R. Br. Rusty Polypod. Fronds 2-8 
inches long ; laminae lanceolate, green above, very chaffy beneath 
