Page:Popular Science Monthly Volume 33.djvu/868

848 at last to appear, that those who read the newspapers and sustain them appreciate matter of solid worth, and will buy it and read it when it is offered to them?



volume deals with one of the most interesting departments in the whole range of botanical science. It is, in fact, almost common ground for both the botanist and entomologist. The author accepts as a fundamental principle that environment furnishes the influence which induces plants to vary. A brief outline of the steps taken by various authorities along this line, from Geoffrey Saint-Hilaire in 1795, to Darwin of recent time and Herbert Spencer of to-day, is given in the preface.

Prof. Henslow early had his attention attracted to floral structures in their relation to insect visitors, and this volume is an elaborate treatise in which the object, in the author's own words, is to "endeavor to refer every part of the structure of flowers to some one or more definite causes arising from the environment taken in its widest sense." The early chapters deal with those elementary principles so essential to a full and accurate understanding of that which follows. Symmetry, or lack of it, is treated at length, and many causes are assigned for the disappearance of petals, stamens, etc., or their augmentation. Then follows a discussion of the principles of arrangement. The alternation of the floral whorls is, for example, considered due to their being composed of spirals "which are projected on to the same plane and so form verticils," and the position of stamens follows in consequence of the branching of fibro-vascular bundles. In some cases the sepaline bundles give rise to a whorl of stamens, and in others the petaline cord. Why this should be is not understood. Nutrition is the immediate cause, but why the nutrition should flow in one or the other direction remains obscure.

The irritation induced by insects is a potent cause of the flow of sap to certain parts, which encourages local growth and thereby brings about a union between parts of a whorl or between different whorls. Prof. Henslow's theory is therefore "that the forms and structures of flowers are the direct outcome of the responsive power of protoplasm to external stimuli." That hypertrophy results from irritation is well shown in many instances, but some persons may be slow in granting all that the author is free to ascribe to the theory. He, however, makes a strong argument, and brings forward a great array of facts. Other causes are, however, not overlooked, and hereditary influences is one of these. Irregularity in flowers is shown to be for the purpose of securing the pollination of the stigma. "All flowers, as we have them now, which are in perfect adaptation to insect agency, are the outcome of the resultant of all the forces, external and internal, which the insect has actually brought into play, or stimulated into action by visiting them for their honey or pollen." With this working theory the author is able to show good reasons for the development of flowers having a bilateral symmetry. The portions of a flower upon which insects alight have become large and strong by responding to the strain that insects have brought upon them. Subsequently hereditary influences have come into play, and now the enlarged part may be present before there is any necessity for it. At the same time compensatory degeneration goes on in other parts of the flower. In the tendency of irregular flowers to become regular under cultivation, the author recognizes negative evidence to his theory. Presuming that the irregularity was brought about by insects, the demand for irregularity under culture being wanting, the flowers revert to their ancient regular form. "Did we but know what the insects were, and how they have poised themselves upon the flower, and in what way their proboscides and tongues have irritated the different parts, one might be able to describe more accurately the whole process; but that such have been the cause and effect as above described, seems to me to be too probable a theory to be hastily discarded in the absence of a better one." The author frequently refers to such striking examples of quick response in tissues to insect irritation as are seen in the formation of galls, and he concludes that if the stimulus 