=565. The sensitive fern.=—This division of labor is carried to an extreme extent in the case of some ferns. Some of our native ferns are examples of this interesting relation between the leaves like the common sensitive fern (Onoclea sensibilis) and the ostrich fern (O. struthiopteris) and the cinnamon-fern (Osmunda cinnamomea). The sensitive fern is here shown in fig. 316. The sterile leaves are large, broadly expanded, and pinnate, the pinnæ being quite large. The fertile leaves are shown also in the figure, and at first one would not take them for leaves at all. But if we examine them carefully we see that the general plan of the leaf is the same: the two rows of pinnæ which are here much shorter than in the sterile leaf, and the pinnules, or smaller divisions of the pinnæ, are inrolled into little spherical masses which lie close on the side of the pinnæ. If we unroll one of these pinnules we find that there are several fruit dots within protected by this roll. In fact when the spores are mature these pinnules open somewhat, so that the spores may be disseminated. [Illustration: Fig. 317. Sensitive fern; one fertile leaf nearly changed to vegetative leaf.] There is very little green color in these fertile leaves, and what green surface there is is very small compared with that of the broad expanse of the sterile leaves. So here there is practically a complete division of labor between these two kinds of leaves, the general plan of which is the same, and we recognize each as being a leaf. [Illustration: Fig. 318. Sensitive fern, showing one vegetative leaf and two sporophylls completely transformed.] =566. Transformation of the fertile leaves of onoclea to sterile ones.=—It is not a very rare thing to find plants of the sensitive fern which show intermediate conditions of the sterile and the fertile leaf. A number of years ago it was thought by some that this represented a different species, but now it is known that these intermediate forms are partly transformed fertile leaves. It is a very easy matter in the case of the sensitive fern to produce these transformations by experiment. If one in the spring, when the sterile leaves attain a height of 12 to 16 _cm_ (8-10 inches), cuts them away, and again when they have a second time reached the same height, some of the fruiting leaves which develop later will be transformed. A few years ago I cut off the sterile leaves from quite a large patch of the sensitive fern, once in May, and again in June. In July, when the fertile leaves were appearing above the ground, many of them were changed partly or completely into sterile leaves. In all some thirty plants showed these transformations, so that every conceivable gradation was obtained between the two kinds of leaves. [Illustration: Fig. 319. Normal and transformed sporophyll of sensitive fern.]