The quaint dictum, "Plants do not grow where they like best, but where other plants will let them," which is credited to the late eminent horticulturalist, Dean Herbert of Manchester, expresses a truth not yet half appreciated by botanists. It is a protest against the prevalent belief that circumstances of climate and soil are the omnipotent regulators of the distribution of vegetables, and that all other considerations are comparatively powerless. The dean's crude axiom has lately found a philosophical exposition and expression in Mr. Darwin's more celebrated doctrine of the "struggle for life, and preservation thereby of the favored races," and if to it we add that great naturalist's more fruitful discovery of the necessity for insect and other foreign agencies in ensuring fertility, and hence perpetuating the species, we shall find that the powers of climate and soil are reduced to comparatively very narrow limits. Before proceeding to show what are the causes that do materially limit the distribution of species, it may be well to inquire how far the hard pressed soil and climate theory really helps us to a practical understanding of one or two great questions that fall under our daily observation; of these, the following are the most prominent.
That very similar soils and climates, in different geographical areas, are not inhabited naturally either by like species or like genera; that very different soils and climates will produce almost equally abundant crops of the same cultivated plants; and that, in the same soil and climate, many hundreds, nay, thousands of species, from other very different soils and climates, may be grown and propagated for an indefinite number of successive generations.
Of the first of these statements, the examples embrace some of the best known facts in geographical botany; as, for example, that the flora of Europe differs wholly from that of temperate North America, South Africa, Australia, and temperate South America, and all these from one another. And that neither soil nor climate is the cause of this difference is illustrated by the fact that thousands of acres in each of these countries are covered, year after year, by crops of the same plant, introduced from one to the other; and by annually increasing numbers of trees, shrubs, and herbs, that have either run wild or are successfully cultivated in each and all of them. The third proposition follows from the two others, and of this the best example is afforded by a good garden, wherein, on the same soil and under identical conditions, we grow, side by side, plants from very various soils and climates, and ripen their seeds too, provided only that their fertilization is insured. The Cape geraniums, London pride, and Lysimachia nummularia in our London areas, the pendent American cacti in the cottage windows of Southwalk and Lambeth, are even more striking examples of the comparative indifference of many plants to good or bad climate or soil; and what can be more unlike their natural conditions than those to which ferns are exposed in those invaluable contrivances, Ward's cases, in the heart of the city? True, the conditions suit them well, and, with respect to humidity and equability of temperature, are natural to them; but neither is the absolute temperature, nor the constitution, nor freshness of the air, the same as of the places the ferns are brought from; nor is any systematic attempt made to suit the soil to the species cultivated; for, as Mr. Ward himself well shows, the arctic saxifrage, the English rose, the tropical palm, and desert cactus live side by side in the same box, and under precisely similar circumstances, and, as it were, in defiance of their natal conditions.
Let it not be supposed that we at all underrate such power as soil and climate really possess. In some cases, as those of chalk, sand, bog, and saline and water plants, soil is very potent; but the number of plants actually dependent on these or other peculiarities of the soil is much more limited than is supposed. Of bonâ-fide water-plants there are few amongst phaenogams. Sand plants, as a rule, grow equally well on stiffer soils, but are there turned out by more sturdy competitors; and with regard to the calcareous soils, it is their warmth and dryness that fits them, to so great an extent, for many plants that are almost confined to them, or are absolutely peculiar to them. So, too, with regard to temperature, there are limits, as regards heat, cold, and humidity, that species will not overstep and live; but, on the other hand, so much has been done by selection in procuring hardy races of tender plants, and so much may be done by regulating the distribution of earth-temperature, etc., that we already grow tropical plants in the open air during a portion of the year, and eventually may do so for longer periods.
Amongst the most striking examples of apparent indifference to natural conditions of soil and climate, I would especially adduce two. One is the Salicornia Arabica, a plant never found in its natural state, except in most saline situations, but which has flourished for years in the Succulent House at Kew, in a pot full of common soil, to which no salt has ever been added; the other is the tea-plant, which luxuriates in the hot, humid valleys of Assam, where the thermometer ranges between 70° and 85°, and the atmosphere is so perennially humid that watches are said to be destroyed after a few months of wear; and it is no less at home in north-western India, where the summers are as hot and cloudless as any in the world, and the winters very cold. I may add that the tea-plant has survived the intense cold of this last January, at Kew, on the same wall where many hardy and half-hardy plants have been killed.
It is, further, a great mistake to suppose that the native vegetation of a country suffers little and very exceptionally by abnormal seasons. The most conspicuous instance of the contrary that ever fell under my observation was the destruction of the gigantic gum-tree (Eucalyptus) forests in the central districts of Tasmania, which occurred, if I remember right, about the year 1837. In 1840, I rode over many square miles of country, through stupendous forests, in which every tree was, to all appearance, absolutely lifeless. The district was totally uninhabited, consisting of low mountain ranges, 2,000 feet above the sea, separating marshy tracts interspersed with broad fresh-water lakes. The trees, much like the great gaunt elms in Kensington Gardens during winter, but much larger, were in countless multitudes, 80 to 180 feet high, close set, and ten to twenty feet in girth; their weird and ghostly aspect being heightened by the fact of most being charred for a considerable distance up the trunk, the effects of the native practice of firing the grass in summer during the kangaroo hunting season; and by the bark above hanging from their trunks in streaming shreds, that waved dismally in the wind; for the species was the stringy-bark gum, that sheds its bark after this fashion. And not only had the gum-trees suffered, but the hardier Leptospermum (tea tree bush) and many others were killed, some to the ground and some altogether; so that, though my journey was in spring and the weather was delightful, the aspect of the vegetation was desolate in the extreme.
In such climates as our own, similar devastations are unknown, and, though we know that our island was once covered with other timber than now clothes it, we have every reason to suppose that the change was slow, and the effect either of a gradually altered climate, or of the immigration of trees equally well or better suited to the conditions of the soil and climate, but which had not previously had the opportunity of contesting the ground with the ruling monarchs of the forest.
Making every allowance, then, for the influence of soil and climate in checking the multiplication of individuals, we have still two classes of facts to account for: the one, that plants which succeed so well, when cultivated, that we are assured both soil and climate are favorable to their propagation, nevertheless become immediately or soon extinct when the cultivator's care is withdrawn; the other, that plants of one country, when introduced into another, even with a very different soil and climate, will overrun it, destroy the native vegetation, and prove themselves better suited to local circumstances than the aboriginal plants of the country. In the first case, the reasons are very various, all of them relating to the conditions of the plant's existence. Of these the two most potent are, the absence of fertilizing agents, and the destruction of seeds and seeding plants. In the present state of our knowledge it is impossible to say which of these is most fatal in its effect. In the case of our annual plants or our cereals, which never run wild, it is the latter certainly; for they seed freely enough; in the case of many perennials, shrubs, and trees, it may be the former, as with the common elm and lime, which rarely or never seed in England, though the latter is so notably frequented by insects during its flowering season; whilst a third cause is to be found in their seedling plants being smothered by others, of which we have numerous examples in our common pasture grasses, which are, perhaps, the most prejudicial in this respect. A most conspicuous example of this is afforded by the common maple, of which the seedlings come up early in spring by thousands in the neighborhood of the parent tree, in lawns and plantations, but scarcely ever survive the smothering effects of the common summer grasses as soon as these begin to shoot.
When I visited the cedar grove on Mount Lebanon, in the autumn of 1860, I found thousands of seedling plants, but every one of them dead; and so effectual is the annual slaughter of the yearlings in that grove, that, though the seeds are shed in millions, and innumerable seedlings annually spring up, there is not a plant in the grove less than about sixty years old. It may hence have been sixty years since a cedar there survived the first year of its existence; that is to say, has struggled through its infancy, and reached the age even of childhood!