If excessive size, in point of organic development, may in general be regarded as a proof of a long protraction of life, special attention is due, among the thalassophytes of the submarine vegetable world, to a species of fucus, Macrocystis pyrifera, Agardh (Fucus giganteus). This marine plant attains, according to Captain Cook and George Forster, a length of 360 feet, and exceeds therefore the height of the loftiest Coniferous trees, not excepting Sequoia gigantea, Endl. (Taxodium sempervirens, Hook, and Arnott) of California.[[MY]] Captain Fitz-Roy has confirmed this statement.[[MZ]] Macrocystis pyrifera grows from 64° south lat. to 45° north lat., as far as the Bay of San Francisco on the north-west coast of the New Continent; indeed Joseph Hooker believes that this species of Fucus advances as far as Kamtschatka. In the waters of the Antarctic seas it is even seen floating between the pack-ice.[[NA]] The cellular band and thread-like structures of the Macrocystis (which are attached to the bottom of the sea by an adhesive organ resembling a claw) seem to be limited in their length by accidental disturbing causes alone.
[83]. p. 220—“Phanerogamic plants already recorded in herbariums.”
Three questions must be carefully distinguished from one another: 1. How many species of plants have been described in printed works? 2. How many of those discovered—that is to say included in herbariums—still remain undescribed? 3. How many species probably exist on the surface of the earth? Murray’s edition of the Linnæan system contains, including cryptogamic plants, only 10,042 species. Willdenow, in his edition of the Species Plantarum from 1797 to 1807, has described as many as 17,457 species of phanerogamia, reckoning from Monandria to Polygamia diœcia. If to these we add 3000 species of cryptogamic plants, we shall bring the number as given by Willdenow to 20,000. More recent investigations have shown how far this estimate of the species described, and of those preserved in herbariums, falls short of the truth. Robert Brown[[NB]] first enumerated above 37,000 phanerogamia, and I at that time attempted to describe the distribution of 44,000 species of phanerogamic and cryptogamic plants, over the different portions of the world already explored.[[NC]] Decandolle finds, on comparing Persoon’s Enchiridium with his Universal System divided into twelve families, that more than 56,000 species of plants may be enumerated from the writings of botanists and European herbariums.[[ND]] If we consider how many new species have been described by travellers since that time, (my expedition alone afforded 3600 of the 5800 collected species of equinoctial plants), and if we bear in mind that there are assuredly upwards of 25,000 phanerogamic plants, cultivated in all the different botanical gardens, we shall soon see how much Decandolle’s estimate is below the truth. From our complete ignorance of the interior of South America (Mato-Grosso, Paraguay, the eastern declivity of the Andes, Santa-Cruz de la Sierra, and all the countries lying between the Orinoco, the Rio Negro, the Amazon, and Puruz), of Africa, of Madagascar, and Borneo, and of Central and Eastern Asia, the idea involuntarily presents itself to the mind that we are not yet acquainted with one third, or probably even with one fifth part of the plants existing on the earth. Drège has collected 7092 phanerogamic species in Southern Africa alone; and he believes that the flora of that region consists of more than 11,000 phanerogamic species, seeing that in Germany and Switzerland, on an equal area (192,000 square miles,) Koch has described only 3300, and Decandolle only 3645 phanerogamia in France. I would here also instance the new genera, consisting partly of high forest trees, which are still being discovered in the neighbourhood of large commercial towns in the lesser Antilles, although they have been visited by Europeans for the last three hundred years. Such considerations, which I purpose developing more fully at the close of this illustration, seem to verify the ancient myth of the Zend-Avesta, that “the creating primeval force called forth 120,000 vegetable forms from the sacred blood of the bull.”
If therefore no direct scientific solution can be afforded to the question, how many vegetable forms—leafless cryptogamia (water algæ, fungi, and lichens), characeæ, liverworts, foliaceous mosses, marsilaceæ, lycopodiaceæ, and ferns—exist on the dry land, and in the wide basin of the sea, in the present condition of the organic terrestrial life of our planet, it only remains for us to employ an approximative method for ascertaining with some degree of probability certain “extreme limits” (numerical data of minima). Since the year 1815, I have, in my arithmetical considerations on the geography of plants, calculated the numbers expressing the ratio which the aggregate of species of different natural families bears to the whole mass of the phanerogamia in those countries where the latter is sufficiently determined. Robert Brown,[[NE]] the greatest botanist of our age, had, prior to my researches, already determined the numerical proportion of the principal divisions of vegetable forms, as for instance of acotyledons (Agamæ, cryptogamic or cellular plants) to cotyledons (Phanerogamia, or vascular plants), and of monocotyledons (Endogenæ) to dicotyledons (Exogenæ). He finds the ratio of monocotyledons to dicotyledons in the tropical zone as in the proportion of 1 to 5, and in the frigid zone, in the parallels of 60° north, and 55° south lat. as 1 to 2½.[[NF]] The absolute numbers of the species are compared together in the three great divisions of the vegetable kingdom, according to the method developed in Brown’s work. I was the first who passed from these principal divisions to the individual families, and considered the number of the species contained in each, in their ratio to the whole mass of phanerogamia belonging to one zone.[[NG]]
The numerical relations of the forms of plants, and the laws observed in their geographical distribution, admit of being considered from two very different points of view. When we study plants in their arrangement according to natural families, without regard to their geographical distribution, the question arises: What are the fundamental forms or types of organization, in accordance with which the greater number of their species are formed? Are there more Glumaceæ than Compositæ on the earth’s surface? Do these two orders of plants combined, constitute one-fourth of the phanerogamia? What numerical relation do monocotyledons bear to dicotyledons? These are questions of general phytology, a science that investigates the organization of plants and their mutual connection, and therefore has reference to the now existing state of vegetation.
If, on the other hand, the species of plants that have been connected together according to their structural analogy, are considered not abstractedly, but in accordance with their climatic relations, and their distribution over the earth’s surface, these questions acquire a totally different interest. We then examine what families of plants predominate in the torrid zone more than towards the polar circle over other phanerogamia? We inquire, whether the Compositæ are more numerous in the new than in the old world, under equal geographical latitudes or between equal isothermal lines? Whether the forms which gradually lose their predominance in advancing from the equator to the poles, follow a similar law of decrease in ascending mountains situated in the equatorial region? Whether the relations of the different families to the whole mass of the phanerogamia differ under equal isothermal lines in the temperate zones on either side of the equator? These questions belong to the geography of plants properly so called, and are connected with the most important problems that can be presented by meteorology and terrestrial physics. Thus the predominance of certain families of plants determines the character of a landscape, and whether the aspect of the country is desolate or luxuriant, or smiling and majestic. Grasses, forming extended Savannahs, or the abundance of fruit-yielding palms, or social coniferous trees, have respectively exerted a powerful influence on the material condition, manners, and character of nations, and on the more or less rapid development of their prosperity.
In studying the geographical distribution of forms, we may consider the species, genera, and natural families of plants separately. A single species, especially among social plants, frequently covers an extensive tract of land. Thus we have in the north, Pine or Fir forests, and Heaths (ericeta); in Spain, Cistus groves; and in tropical America, collections of one and the same species of Cactus, Croton, Brathys, or Bambusa Guadua. It is interesting to study more closely these relations of individual increase, and of organic development; and here we may inquire, what species produces the greatest number of individuals in one certain zone; or, merely what are the families to which the predominating species belong in different climates. In a very high northern latitude, where the Compositæ and the Ferns stand in the ratios of 1 : 13 and 1 : 25 to the sum of all the phanerogamia (i. e., where these ratios are found by dividing the sum total of all phanerogamia by the number of species included in the family of the Compositæ, or in that of the Ferns); one single species of Fern may, however, cover ten times more space than all the species of the Compositæ taken together. In this case the Ferns predominate over the Compositæ by their mass, and by the number of the individuals belonging to the same species of Pteris, or Polypodium; but they will not be found to predominate, if we only compare the number of the different specific forms of the Filices, and of the Compositæ, with the sum total of all Phanerogamia. As, therefore, multiplication of plants does not follow the same laws in all species, and as all do not produce an equal number of individuals, the quotients obtained by dividing the sum of all phanerogamic plants by the species of one family, do not alone determine the leading features impressed on the landscape, or the physiognomy of nature peculiar to different regions of the earth. If the attention of the travelling botanist be arrested by the frequent repetition of the same species, by its mass, and the uniformity of vegetation thus produced, it will be still more forcibly arrested by the infrequency of many other species useful to man. In tropical regions, where the Rubiaceæ, Myrtles, Leguminosæ, or Terebinthaceæ, compose the forests, one is astonished to meet with so few trees of Cinchona, or of certain species of mahogany (Swietenia), of Hæmatoxylon, Styrax, or balsamic Myroxylon. I would also here refer to the scanty and detached occurrence of the precious febrifuge-bark trees (species of Cinchona) which I had an opportunity of observing on the declivity of the elevated plains of Bogota and Popayan, and in the neighbourhood of Loxa, in descending towards the unhealthy valley of the Catamayo, and to the river Amazon. The febrifuge-bark hunters (Cazadores de Cascarilla), as those Indians and Mestizoes are called at Loxa, who each year collect the most efficacious of all the medicinal barks, the Cinchona Condaminea, among the lonely mountains of Caxanuma, Uritusinga, and Rumisitana, undergo considerable danger in climbing to the summits of the highest forest-trees, in order to obtain an extended view, from which they may distinguish the scattered, slender, and aspiring trunks of the Cinchona, by the reddish tint of their large leaves. The mean temperature of this important forest region (between 4° and 4½° south lat.) varies from 60° to 68° Fahr., at an absolute height of from 6400 to 8000 feet above the level of the sea.[[NH]]
In considering the distribution of species, we may also, independently of individual multiplication and mass, compare together the absolute number which belong to each family. Such a mode of comparison, which was employed by Decandolle,[[NI]] has been extended by Kunth to more than 3300 of the species of Compositæ with which we are at present acquainted. It does not show what family preponderates by individual mass, or by the number of its species, over other phanerogamic forms, but it simply indicates how many of the species of one and the same family are indigenous in any one country or portion of the earth. The results of this method are, on the whole, more exact, because they are obtained by a careful study of the separate families, without requiring that the whole number of the phanerogamia of every country should be known. Thus, for instance, the most varied forms of Ferns are found in the tropical zone, each genus presenting the greatest number of species in the temperate, humid, and shaded mountainous parts of islands. While these species are less numerous in passing from tropical regions to the temperate zone, their absolute number diminishes still more in approaching nearer to the poles. Although the frigid zone, as, for instance, Lapland, supports species of the families which are best able to resist the cold, Ferns predominate more over other phanerogamia in Lapland than either in France or Germany, notwithstanding the absolute inferiority of the gross number of ferns indigenous to the northern zone, when compared with other countries. These relations are, in France and Germany, as ¹⁄₇₃ and ¹⁄₇₁, while in Lapland they are as ¹⁄₂₅. These numerical relations (obtained by dividing the sum total of all the phanerogamia of the different floras by the species of each family) were published by me in 1817, in my Prolegomena de distributione geographica Plantarum, and corrected in accordance with the great works of Robert Brown, in my Essay on the Distribution of Plants over the earth’s surface, which I subsequently wrote in French. These relations, as we advance from the equator towards the poles, necessarily vary from the ratios obtained by a comparison of the absolute number of the different species belonging to each family. We often see the value of the fractions increase by the decrease of the denominator, whilst the absolute number of the species is reduced. In the fractional method which I have followed as the most applicable to questions relating to the geography of plants, there are two variable quantities; for in passing from one isothermal line to another, we do not find the sum total of the phanerogamia change in the same proportion as the number of the species of one particular family.
In proceeding from the consideration of these species to that of the divisions established in the natural system according to an ideal series of abstractions, we may direct our attention to genera or races, to families, or even to still higher classes of division. There are some genera, and even whole families, which exclusively belong to certain zones; not merely because they can only thrive under a special combination of climatic relations, but also because they first sprang up within very circumscribed localities, and have been checked in their migrations. The larger number of genera and families have, however, their representatives in all regions of the earth, and at all elevations. The earliest inquiries into the distribution of vegetable forms had reference to genera alone, and are to be found in the valuable work of Treviranus.[[NJ]] This method is, however, less appropriate for yielding general results, than that which compares the number of the species of each family, or the great leading divisions (acotyledons, monocotyledons, and dicotyledons), with the sum total of the phanerogamia. In the frigid zone, the variety of forms, or the number of the genera, does not decrease in an equal degree with that of the species, there being in these regions relatively more genera and fewer species.[[NK]] The case is almost the same on the summits of high mountain-chains, where are sheltered individual members of many different genera which one would be disposed to regard as belonging exclusively to the vegetation of the plain.
I have deemed it expedient to indicate the different points of view from which the laws of the distribution of vegetable forms may be considered. It is only when these points of view are confounded together, that we meet with contradictions, which have been unjustly attributed to uncertainty of observation.[[NL]] When expressions like the following are employed: “This form, or this family diminishes as it approaches towards the cold zone,” or “the true habitat of this form is in such or such a parallel of latitude;” or “this is a southern form,” or, again, “it predominates in the temperate zone;” it should be definitely stated whether reference is made to the absolute number of the species, and the proportion of their predominance according to the increase or decrease of latitude; or whether the meaning conveyed is, that a family, when compared with the whole number of the phanerogamia of a flora, predominates over other families of plants. The impression conveyed to the mind of the predominance of forms, depends literally on the conception of relative quantity.