Page:Popular Science Monthly Volume 45.djvu/809

Rh throughout the genus, we may assume the ancestral herbaceous barberry to have had the same peculiarity, but how this remarkable degree of sensitiveness could have arisen is not so clear. It would seem as if insect agency in some way or other must have brought about a movement having such an obviously purposeful relation to insect visits; but when we reflect upon the almost universal absence of a similar movement in flowers similarly visited, and the very questionable usefulness of the pronounced irritability of "sensitive" leaves, it is apparent that such a simple general explanation really explains very little. The few conjectures that the writer has to offer on the subject will be best understood after we have considered what may probably have been the evolution of certain structural peculiarities of the flower.

The anthers (Fig. 3, A), opening as they do by little valves hinged at the top, present a form of dehiscence confined entirely to the Berberidaceæ, the Lauraceæ, and a few other nearly related families not represented in our native flora. Within the Berberidaceæ all the genera except Podophyllum (the May apples) and Nandina have the anthers thus characterized; hence, it is clear that the stamens of the ancestral berberis were already of this peculiar type, and so the antecedent stages should be thought of as occurring in that line of berberidaceous herbs which were the forerunners of the barberries. The herbaceous genus Podophyllum contains species exhibiting a difference in the stamens which affords us an important clew for the understanding of what these antecedent stages may have been like. In P. Emodi (Fig. 13, A) the dehiscence of the anthers is by a longitudinal slit down the middle of each lobe. In P. peltatum (B), our common species, there is likewise a vertical slit, but it is so near the inner side of the connective that there appears to be but one valve to each lobe, the other inner valve having been reduced to a mere vestige. To connect this condition with that common elsewhere in the family, we need only suppose the attachment of the enlarged valve to become gradually narrowed by a continuation of the slit from below (C) until there remains only the small hinge we find in the barberry stamens of to-day (D). It deserves notice that the hinge, instead of being quite at the top, is nearer the back of the anther, which is what might be expected according to the hypothesis.

In the other families we have mentioned as exhibiting a valvular dehiscence of the anthers there is found almost invariably a pair of nectar glands on each filament (see Fig. 15). Now, it is a curious fact that in certain of the less highly developed mahonias the filaments are each provided with a pair of appendages (Fig. 14, A) similarly placed but being, so far as we know.