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DISTRIBUTION] structure. If this be so, it may justifiably be inferred that both normal and abnormal morphological features may be due to the presence of enzymatic substances secreted by the protoplasm that determine the course of development. At any rate this hypothesis suggests an explanation of many hitherto inexplicable facts. For instance, it has been pointed out in the article on the reproduction of plants that the effect of the fertilization of the female cell in the ovule of a phanerogam is not confined to the female cell, but extends more or less widely outside it, inducing growth and tissue-change. The ovule develops into the seed; and the gynaeceum and even more remote parts of the flower, develop into the fruit. The facts are familiar, but there is no means of explaining them. In the light of Sachs's theory the interpretation is this, that the act of fertilization causes the formation in the female cell of substances which are transmitted to adjacent structures and stimulate them to further development.

.—As the scope of this article limits it to the general principles of the morphology of plants, comparatively few facts have been adduced. Full morphological and organographical details are given in the articles on the various groups of plants, such as those on the Algae, Bryophyta, Pteridophyta, Angiosperms, Gymnosperms, &c. The following works may also be consulted: Schimper, Plant-Geography (Clarendon Press, Oxford); Goebel, Organography (Clarendon Press, Oxford); Bower, The Origin of a Land Flora (Macmillan); Beyerinck, “Ueber Cecidien,” (Bot. Zeitung, 1888). (S. H. V*)

Common experience shows that temperature is the most important condition which controls the distribution of plants. Those of warmer countries cannot be cultivated in British gardens without protection from the rigours of winter; still less are they able to hold their own unaided in an unfavourable climate. Temperature, then, is the fundamental limit which nature opposes to the indefinite extension of any one species. Buffon remarked “that the same temperature might have been expected, all other circumstances being equal, to produce the same beings in different parts of the globe, both in the animal and vegetable kingdoms.” Yet lawns in the United States are destitute of the common English daisy, the wild hyacinth of the woods of the United Kingdom is absent from Germany, and the foxglove from Switzerland. We owe to Buffon the recognition of the limitation of groups of species to regions separated from one another by “natural barriers.” When by the aid of man they surmount these, they often dominate with unexpected vigour the native vegetation amongst which they are colonists. The cardoon and milk thistle, both European plants, cover tracts of country in South America with impenetrable thickets in which both man and beast may be hopelessly lost. The watercress blocks the rivers of New Zealand into which it has been introduced from Europe. The problem, then, which plant distribution presents is twofold: it has first to map out the earth's surface into “regions” or “areas of vegetation,” and secondly to trace the causes which have brought them about and led to their restriction and to their mutual relations.

The earliest attempts to deal with the first branch of the inquiry may be called physiognomical. They endeavoured to define “aspects of vegetation” in which the “forms” exhibited an obvious adaptation to their climatic surroundings. This has been done with success and in great detail by Grisebach, whose Vegetation der Erde from this point of view is still unsurpassed. With it may be studied with advantage the unique collection at Kew of pictures of plant-life in its broadest aspects, brought together by the industry and munificence of Miss Marianne North. Grisebach declined to see anything in such “forms” but the production by nature of that which responds to external conditions and can only exist as long as they remain unchanged. We may agree with Schimper that such a point of view is obsolete without rejecting as valueless the admirable accumulation of data of which it admittedly fails to give any rational explanation. A single example will be sufficient to illustrate this. The genus Senecio, with some 1000 species, is practically cosmopolitan. In external habit these exhibit

adaptations to every kind of climatic or physical condition: they may be mere weeds like groundsels or ragworts, or climbers masquerading like ivy, or succulent and almost leafless, or they may be shrubs and even trees. Yet throughout they agree in the essential structure of their floral organs. The cause of such agreement is, according to Grisebach, shrouded in the deepest obscurity, but it finds its obvious and complete explanation in the descent from a common ancestor which he would unhesitatingly reject.

From this point of view it is not sufficient, in attempting to map out the earth's surface into “regions of vegetation,” to have regard alone to adaptations to physical conditions. We are compelled to take into account the actual affinity of the plants inhabiting them. Anything short of this is merely descriptive and empirical, and affords no rational basis for inquiry into the mode in which the distribution of plant-life has been b1ought about. Our regions will not be “natural” unless they mark out real discontinuities both of origin and affinity, and these we can only seek to explain by reference to past changes in the earth's history. We arrive thus at “the essential aim of geographical botany,” which, as stated by Schimper, is “an inquiry into the causes of differences existing among the various floras.” To quote further: “Existing floras exhibit only one moment in the history of the earth's vegetation. A transformation which is sometimes rapid, sometimes slow, but always continuous, is wrought by the reciprocal action of the innate variability of plants and of the variability of the external factors. This change is due partly to the migrations of plants, but chiefly to a transformation of the plants covering the earth.” This transformation is due to new characters arising through variation. “If the new characters be useful, they are selected and perfected in the descendants, and constitute the so-called ‘adaptations’ in which the external factors acting on the plants are reflected.” The study of the nature of these adaptations, which are often extremely subtle and by no means merely superficial, is termed Ecology (see above).

The remark may conveniently find its place here that plants which have reached a high degree of adaptive specialization have come to the end of their tether: a too complicated adjustment has deprived them of the elasticity which would enable them to adapt themselves to any further change in their surroundings, and they would pass away with conditions with which they are too inextricably bound up. Vast floras have doubtless thus found their grave in geologic change. That wrought by man in destroying forests and cultivating the land will be no less effective, and already specimens in our herbaria alone represent species no longer to be found in a living state. Extinction may come about indirectly and even more surely. This is easy to happen with plants dependent on insects for their fertilization. Kronfeld has shown that aconites are dependent for this on the visits of a Bombus and cannot exist outside the area where it occurs.

The actual and past distribution of plants must obviously be controlled by the facts of physical geography. It is concerned with the land-surface, and this is more symmetrically disposed than would at first sight appear from a glance at a map of the world. Lyell points out that the eye of an observer placed above a point between Pembroke and Wexford, lat. 52° N. and long. 6° W., would behold at one view the greatest possible quantity of land, while the opposite hemisphere would contain the greatest quantity of water. The continental area is on one side of the sphere and the oceanic on the other. Love has shown (Nature, Aug. 1, 1907, p. 328) that this is the result of physical causes and that the existence of the Pacific Ocean “shows that the centre of gravity of the earth does not coincide with the centre of figure.” One half of the earth has therefore a greater density than the other. But “under the influence of the rotation the parts of greater density tend to recede further from the axis than the parts of less density. . . the effect must be to produce a sort of furrowed surface.” The furrows are the great ocean basins, and these would still persist even if the land surface were enlarged to the 1400 fathoms contour. These considerations