sensitive fern has a running rootstock, scattered fronds, and netted veins;
while the ostrich fern has an upright rootstock, fronds in crowns, and
free veins.

[Illustration: Sensitive Fern. Gradations from Leaf to Fruit.
_Obtusilobata_ Form]

(1) SENSITIVE FERN. _Onoclèa sensíbilis_

Fronds one to three feet high, scattered along a creeping rootstock,
broadly triangular, deeply pinnatifid, with segments sinuately lobed or
nearly entire. Veins reticulated with fine meshes. The fertile fronds
shorter, closely bipinnate with the pinnules rolled up into berry-like
structures which contain the spore cases. (The name in Greek means a closed
vessel, in allusion to the berry-like fertile segments.) The sensitive
fern is so called from its being very sensitive to frost. The sterile and
fertile fronds are totally unlike, the latter not coming out of the ground
until about July, when they appear like rows of small, green grapes or
berries, but soon turn dark and remain erect all winter, and often do not
discharge their spores until the following spring. The little berry-like
structures of the fertile frond represent pinnules, bearing fruit-dots,
around which they are closely rolled. As Waters remarks, "Most ferns hold
the sori in the open hand, but the sensitive fern grasps them tightly in
the clenched fist."

Var. _obtusilobatà_ is an abortive form with the fertile segments only
partially developed. The illustration shows several intermediate forms.

[Illustration: Sori of Sensitive Fern]