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angiosperms
hairs, as well as of the carrying vasa; for the latter there are some of the Pteridophytes, but unlike these they do is great variety of method in relation to other oecological not spread by the scattering of the megaspores. Gamo. On the contrary the megasporangium remains genetic factors. There may be a reduction in the amount of the persistently closed and attached to the spori- reproductranspiration-surface by the shortening and thickening of plant, whilst the megaspore which it t,on’ the stem and the foliage-leaves, or even by the suppression ferous contains (for there is commonly but one) germinates of the latter (aphylly) leading to stem-assimilation, the within it and produces the structures which are the homodevelopment of cladodes, and the production of nontranspiring thorns, while by the formation of vertical logues of the female gametophyte of the Pteridophytes its female gametes. Nor does this intrasporal leaves and phyllodes the effect of insolation is. restricted. with Obstacles to transpiration are created by increase in development cease here. Conjugation of the male and thickness and in cutin - content of the outer wall of the female gametes takes place within the spore, as does also epidermal cells, by the sinking of the stomata, by the the subsequent segmentation of the resulting zygote, and development of a covering of hairs, or in compound leaves its further development through the stage of pro-embryo by collapsing movements. In many cases water-storage of to the formation of the perfect embryo of a sporophyte. kinds is provided, from which loss may be supplied and There is thus produced in the position of the original megasucculence of stem (chylocauly) or of leaf (chylophylly) is sporangium a body consisting, in its simplest form, of a developed. An increase in the sclerenchyma-tissue of coat to the outside composed of the wall of the megastem (sclerocauly) or of leaf (scleropliylly) and an elonga- sporangium and of the wall of the megaspore, both of them tion of the palisade-cells of the leaf are also accompani- much altered, and within of an embryo sporophyte. This ments of this adaptation. Such adaptations are termed body is the seed. For a longer or shorter time it remains xerophilous, and plants exhibiting them are Xerophytes. attached to the sporiferous plant, and its ultimate separaThe conditions which induce the xerophilous structure are tion provides for the spread of the angiospermous plant. conspicuously and most typically found in desert areas, It is this character—of a closed megasporangium and the but they also, in varying degree, surround plants growing development of seed—which separates sharply the Angioon the surface of rocks {Liihophytes), in rock-fissures sperms and the Gymnosperms from all other classes in the (Chasmophytes), on sandy porous ground (Psammophytes), plant kingdom, and which makes so apt their designation on the shore {Halophytes), on peat-moors {Ericophy tes), in one group—the Spermophytes. The exigencies of their and elsewhere. In marked contrast with xerophilous adaptation as a land-vegetation have made the developconfiguration and structure are the hygrophilous features ment of sexual cells in them not a first charge upon an which characterize Hygrophytes or plants j independent or nearly independent organism arising from Hygrophily. wYiich are not, or only rarely and temporarily, a spore, as is the case in the Pteridophytes, but these liable to any danger from deficiency of water-supply. appear as late products on a vast vegetative structure Here the presence of a feeble absorptive root-system and through which the outcome of their coalescence, the an extended surface of the shoot for transpiration and embryo, can be nourished for a time, and from which it is transudation are the outstanding points. The latter is subsequently shed, well protected in the seed-coat to exhibited in the elongated stems which do not bear thorns, withstand climatic vicissitudes yet ready to take its although prickles are not uncommon, the large surface and position as a new plant whenever environmental conditions the thinness of the leaves which frequently have “ dropper permit. The risk of failure of sexual reproduction through tips” or other arrangements for quickly throwing off water, non-propinquity of the essential energids overhangs the the large air canals, abundant and exposed stomata, and higher Pteridophytes at two stages of their life history, many hydathodes. The risk to hygrophytes lies in their and frequent actual failure may have hastened their retroexposure to strong insolation or to prolonged air currents, gression as a dominant vegetative type j there is first of all and these may cause flagging of the organs; but loss from that of non-association on the same nidus of the fortuitously such causes is soon recovered, and may be provided against distributed microspores and megaspores, and second that by water-storage in or near the epidermis. Hygrophytes of a missing of coalescence of the male and female occur in moist areas, and are particularly abundant in the gametes, inasmuch as the act involves a sufficiency of tropics. The degree to which xerophilous or hygrophilous ! moisture for germination, and through which the male construction is developed varies, and some plants exhibit gamete must actively move to its goal. In the Angiosperms this risk is, to their advantage, reduced to a single one, seasonal alternation of these characters, and Tropophily. jaiave therefore termed Tropophytes. In that of a mishap in pollination, for the avoidance of which the wet or warm season they are hygrophilous, in the dry have been developed the elaborate contrivances which make or cold season they are xerophilous. The feature dis- up the flower of to-day. The part of the Angiosperm through which the formatinguishing them is the casting of the greater portion of their transpiration-surface at the beginning of the period tion of seed takes place is the flower. This is a shoot, when there is a risk of physiological drought, by the bearing sporangia which contain spores, both flower. dying down of the annual shoots of herbs, and by leaf-fall sporangia and spores being concerned in the proand branch-fall of shrubs and trees. That this is a pre- duction of the seed. Function as well as structure paration for physiological drought every gardener may is therefore involved in the conception of flower. Sporansee when he transplants such an evergreen as a holly. De- giferous shoots resembling those of some Angiosperms are ciduous-leaved trees and shrubs of temperate regions have found outside the Spermophytes in, for example, Selagihygrophilous foliage, but the stems are xerophilous, being nella, and they have the structural features of flower, but protected by cork, and the buds are enveloped in scales, they do not form seed, and are therefore not flower. resin, and so forth. Evergreens of temperate regions have Nowhere, except in the Gymnosperms and Angiosperms, xerophilous leaves, their young leaves alone showing hygro- do we find such sporangiferous shoots implicated in the philous characters. Any one of the growth-forms—herb, physiological processes which result in the seed. The shrub, tree, liane — may exhibit features of xerophily, microsporangia {pollen-sacs) containing microspores {pollenhygrophily, or tropophily, and their adaptations upon grains) and the megasporangia {ovules) containing megathese lines determine the physiognomy of vegetation spores {embryo-sacs) are enclosed in leaves, respectively, the microsporophylls {stamens) and the megasporophylls within the thermal zones of the globe. Angiosperms (like Gymnosperms) are heterosporous, as {carpels) attached to the axis {torus) of the flower.
