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angiosperms
class Didynamia. Its use with any approach to its modern scope only became possible after Robert Brown had established, in 1825, the existence of truly naked seeds in the Cycadese and Comferse entitling them to be correctly called Gymnosperms. From that time onwards, so long as these Gymnosperms were as was usual, reckoned as dicotyledonous flowering plants, the term Angiosperm was used antithetically by botanical writers, but with varying limitation, as a group-name for other dicotyledonous plants. The advent m 185i of Hofmeister’s brilliant discovery of the changes proceeding in the embryo-sac of flowering plants, and his determination of the correct relationships of these with the Cryptogamia, fixed the true position of Gymnosperms as a class distinct from Dicotyledones, and the term Angiosperm then gradually came to be accepted as the suitable designation for the whole of the flowering plants other than Gymnosperms, and as including therefore the classes of Dicotyledones and Monocotyledones. This is the sense in which the term is nowadays received and m which it is used here. The Angiosperms and the Gymnosperms together compose the group of Spermophytes also named Phanerogams—Seed Plants or Flowering Plants. The expressions seed plant and flowering plant convey the essential character of the group. The seed is the product of the flower—the conception of the flower implies that of the seed. In no other group of plants is there a seed or a flower, although the terms are sometimes loosely used with reference to plants of lower organization. The trend of the evolution of the plant kingdom has been in the direction of the establishment of a vegetation of fixed habit and adapted to the vicissitudes of a life on land, and the Angiosperms are the highest expression of this evolution and constitute the dominant vegetation of the earth’s surface at the present epoch. There is no land-area from the poles to the equator, where plantlife is possible, upon which Angiosperms may not be found. They occur also abundantly in the shallows of rivers and fresh-water lakes, and in less number in salt lakes and in the sea; such aquatic Angiosperms are not, however, primitive forms, but are derived from immediate land-ancestors. The varying climatic or environmental conditions to which Angiosperms may be (Ecology. eXp0se(j their wide distribution, including
ment alone. These categories are not altogether sharply distinguishable, and many plants have the capacity of dual adaptation; e.g., Batrachian Ranunculi may be either hydrophilous or geophilous; Ficus bengalensis as a juvenile is aerophilous, but geophilous when adult. I. Geophytes.—Geophytes overwhelmingly predominate in the world, and from them we derive our concept of the typical Angiosperm, which for the performance of its life-work has two series of vege- orsan-tza. tative organs severally adapted to the media— tion% soil and atmosphere—in which they are exposed. They are the root or descending system and the shoot or ascending system composed of stem and leaf. To the root belongs the special work of fixing the plant in the soil and of absorbing material for food in a liquid state. To the shoot is assigned the special task of taking in material for food in a gaseous form from the atmosphere and of absorbing the radiant energy of the sun. The primary root formed at the base of the embryo pierces the soil as a main radially-constructed orthotropous root, from which lateral endogenetic plagiotropous rootlets branching like the mother proceed. Roots elongate by means of an intercalary pluricellular growing point covered by a protecting root-cap, and bear over a short distance close behind the point a covering ot short-lived active hairs, the agents alike of fixing and absorbing.
those of the soil, edaphic, those of the atmosphere, epedaphic, and those of water, aquatic, find their response, within the limits of phylogeny, in the external form and internal structure exhibited by the plants. Angiosperms of a tropical forest, for example, are manifestly of different form from those of an alpine region, or again Angiosperms growing submerged in a lake have features readily separating them from plants of a desert plain; the differences are not confined to points of their outward form, but obtain throughout their internal structure. The study of the relationships of Angiosperms, as well as of other plants, to their environment has in recent years become a fascinating and productive field of botanical work to which has been given the designation “ GEcology ” (sometimes written “ Ecology ”). From the standpoint of their relation to environment Angiosperms fall into the three main categories of Geophytes, Aerophytes, Hydrophytes. Geophytes are plants adapted to a land-life with fixation in the soil, and therefore they are subject to the influence of both edaphic and epedaphic factors of environment. Aerophytes are adapted to a land-life with fixation upon another plant, and are therefore subject to the influence of epedaphic factors of environment only. Hydrophytes, being adapted to life in water, whether free or fixed, are subject to the influence of the aquatic environ-
The depth to which the root-system of Angiosperms penetrates the soil is apparently conditioned by the needs of respiration, and even in plants with highest shootdevelopment in temperate regions is not over 4 feet, save in exceptional conditions. The growth in length of the main root is therefore soon arrested. The horizontal extension of the lateral roots is practically indefinite. The elongation and persistence of the primary root is a characteristic feature of dicotyledonous Angiosperms, and is in marked contrast with what is commonly found in Monocotyledones and in Pteridophytes, and is evidently correlated with their more elaborate development of shoot. Where the primary root elongates in Monocotyledones it rarely persists for any time, even in cases where there is a considerable bulk in the shootsystem ; the root-system is then provided by adventitious roots from the stem. The primary shoot-axis has two portions—the hypocotyl with its cotyledon or cotyledons, the portion first formed in the embryo, and the epicotyl, which in the embryo exists as the plumular bud. The latter elongates as a radial orthotropous axis, with a terminal pluricellular growing point of indefinite growth covered more or less by incipient leaves, both scale-leaves and foliage-leaves, to form a leaf-bud, and these arise from nodal points as exogenetic outgrowths, commonly dorsiventral and of limited intercalary basipetal growth. Branches like the parents, but plagiotropous and often dorsiventral, arise exogenetically in the axlls of the leaves. The leaves (as also their axillary branch) have a definite position—spiral or cyclic—upon the stem. The degree to which the shoot is developed and its duration above ground have given origin to the popular designations herb, shrub, tree, and these, terms, Growttt_ although not capable of scientific definition, for /orms. the forms they indicate pass one into the other, are nevertheless landmarks in outward form development growth-forms—in relation to climatic factors, and aie therefore of oecological value. To them may be added liane, as the designation of a subsidiary form of widespread occurrence. A herb being a plant annual or perennial—which loses and renews its shoot-growth from year to year, and often also loses and renews its rootsystem, does not rise to any great height above the soil. It is therefore not greatly exposed to air-currents, and it may for a considerable portion of the year be subterranean and consequently under more equable conditions than when above ground; it dwells in a moist stratum of the
