ANGIOSPERMS carried by water-currents, serve as a means of distribution. To this circumstance is probably due the wide area of distribution of many hydrophytes. A typical Angiosperm is an autotrophic (prototrophic) plant. By this we mean that it is able to build up its Autotro- organic food-material within itself from inorganic phism and substances taken by it from without. In the heterotro- early juvenile phases of their life, however, phism. many such autotrophic Angiosperms are heterotrophic, that is to say, they take in their organic foodmaterial from without, inasmuch as they feed in germination upon reserve-food stored up within the seed, frequently by means of a special sucker produced by the cotyledon. This is after all only a case of prolongation of the nursing process common to all embryos, and of the drawing of food from one set of cells to another which is characteristic of the higher plants in consequence of their colonial organization. On the other hand some Angiosperms are persistently heterotrophic, either as saprophytes (metatrophic) or as parasites (paratrophic), and this manner of life involves some kind of symbiosis. Between truly autotrophic Angiosperms and truly heterotrophic ones there are all degrees, and such intermediate forms have been designated mixotrophic. A typical saprophyte is a plant which derives its foodmaterial from dead organic matter. This method of feeding is associated with the presence of the mycelium of a S P absorbing system and formp hyt°~sm. fungus yCOrfiiza> toandthethere is a reciprocal symbiosis a mattached between the Angiosperm and the fungus which is endophytic, only rarely ectophytic. Reduction of the vegetative system of the Angiosperm accompanies this. The subterranean portion of the plant is small and the roots may be absent. In the shoot chlorophyll is suppressed, and commonly a brown pigment, the use of which is unknown, is present; there are only scale leaves, and the carrying system is rudimentary. Such typical saprophytes are holosaprophytes. Where chlorophyll is present in small amount the plant is able to manufacture some at least of its carbonaceous food and is then a hemisayrophyte. Angiospermous holosaprophytes are not common. They are known amongst Gentianeae and Monotropese only of Dicotyledones, and most of them are Monocotyledones of the Orchideae, Burmanniaceae, and Triurideae. They are derived from autotrophic forms and do not appear to be associated with any special climatic conditions, but, as their structure would suggest, are most of them plants of shade and moisture. A large number of Angiosperms which appear to be independent autotrophic forms are now known to engage in a form of symbiosis, the exact relations in which are not yet determined. More or fewer of their rootlets have their extremities invested by a weft of hyphal mycelium as an ectophytic mycorhiza. This is not constant in one species, nor do all rootlets of one plant show it. Its frequency, however, indicates some reciprocal relation between the Angiosperm and the fungus. The occurrence of mycodomatia upon the roots of plants of Leguminosse as well as of other families is a result of a symbiosis, and is of supreme importance in agriculture on account of the accumulation of nitrogenous material that takes place within them. The fact known to gardeners that, as the wood of tubs in which large plants are grown gradually decays, the roots of the occupant spread into it and there develop mycorhiza, usually endophytic, points in the direction ot the facultative saprophytism of all Angiosperms. The habit possessed by a few plants of different cycles of affinity of capturing insects in traps of various kinds, enables them to obtain an accession to their supply of nitrogen. In some of them a proteolytic enzyme brings about the digestion of the insect for the benefit of the plant (Droseracese, Nepenthacese), in other cases there appears to be no enzyme, and the plant absorbs soluble nitrogenous substances formed in normal putrefaction (Sarraceniacese). Some carnivorous plants are hydrophytes (Utricularia). The flowerstructure of typical saprophytes has no feature of special interest, but it is much to be wished that we had full knowledge of the changes in the embryo-sac and of the phases of embryogeny. A parasite takes all or a portion of its food-material from a living host into which it penetrates. The degree of connexion the two and the dependence of the parasite Parasitism. between Up0n vary between the completeness of that of holoparasitic Rhizanths, in which little more than the flower of the parasite is visible upon the outside of the stem of the host and the parasitism is absolute, and the limitation of that of the
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hemiparasitic Rhinanthese, in which to all appearance there is an independent autotrophic geophyte, for it possesses chlorophyll and its union with the host is confined to isolated points below ground. The seat of attachment of the parasite may be either the root or the stem of the host; sometimes, in the case of twining parasites, the leaf may also be made use of. In the simplest cases the parasite develops a sucker (haustorium), from the centre of which piercing connexions of kinds proceed, and it then maintains its individual shoot development, which may be that of a herb (annual or perennial), shrub, tree, or liane, outside the host. But in the more advanced conditions of parasitism the reduction of the vegetative system of the parasite is so great that its individuality outside the host is lost, and it appears as a^web of threads running within and blending with the tissues of the host, in the manner of a mycelium, or forming with the tissue of the host a tuberous conjoint stock from which the reproductive system shoots. All angiospermous parasites are derived from autotrophic forms, but the modification in the form and structure of their organs—both vegetative and reproductive—induced by the parasitism, makes the relation of holoparasitic groups somewhat obscure. Parasitism is unknown amongst Monocotyledones, but has appeared more than once amongst Dicotyledones. Cytinese, Balanophorete, Orobanchacese, Lennoacese, are families of differing position which are characteristically holoparasitic. Loranthacese and Santalacese are chiefly hemiparasitic, as are most of the parasitic Scrophularineae. Parasitism is not associated with any special epedaphic conditions, and there are no hydrophilous parasites. It occurs in all regions of the world, and in the varying environment of dark damp tropical woods, arid plains of warm regions, tropophil woods of temperate regions, and alpine slopes. Accordingly both xerophilous and hygrophilous features appear. The host in some cases seems to have no particular attraction to the parasite, and any species of plant may be used by the same specific parasite, even its own body being acceptable ; on the other hand, there are parasitic species which are only known upon one kind of host. Of the factors which co-operate in bringing about the attachment of a parasite to its host we know nothing beyond the fact that chemiotactic and contact stimuli are, as in all cases of infection, concerned in it. The life of the host may or may not be affected by the parasite. Species of Cuscuta, if they do not kill, may so arrest the growth of their host-plant as to bring about results disastrous in agriculture, and by the penetration of loranthaceous plants deformities of stem and branches are produced in trees, which then become useless for timber. It has been claimed that in the case of chlorophyllaceous parasites, the symbiosis may not be antagonistic but reciprocal, but even if this be so to some extent, the amount of aid rendered to the host does not compensate for the drain upon its resources made by the parasite. So conspicuous is this that legislation restricting the cultivation of mistletoe, on account of the damage caused by it in orchards, has been deemed necessary in some districts in Europe. In the more recently derived parasites, those of bicarpellate gamopetalous Dicotyledones, most of which are hemiparasitic and epirhizal, the flower-structure and seed-formation retain in the main their immediate ancestral features, but in the families of the Loranthales, to which all evidence concurs in ascribing a far back origin, many modifications in the way of reduction are found. The inflorescence may be endogenetic, the ovule may consist of nucellus alone, and frequently there is no ovule—the embryo-sacs, of which there are many, being then developed in the torus or in the ovarian wall. The formation of endosperm offers many features of interest, one of the most remarkable being that seen in Balanophora, where the apical polar energid alone of the energids within the embryo-sac survives disintegration, and then, in the absence of fertilization, segments to form a pluricellular endosperm upon which an embryo apogamously buds. The history of few forms is well known, and these require re-study in the light of the recently - acquired knowledge of fertilization. There is no more promising field for investigation than that of the embryogeny of parasites. Intimate relations between Angiosperms and the animal kingdom have been shown to exist in the work of pollination, of dissemination, and of the feeding of carni- Myrmecopbily. vorous plants. Another relation of which the facts are clear enough, but of which the evolution is doubtful, is that known as Myrmecophily. Species of ants make their home on tropical plants, living in hollows of stem, petiole, or stipule, which are commonly enlarged and may form tuberous cushions. They feed upon either the secretion of extra-floral nectaries, or upon the richly nitrogenous contents of modified glands (Muller’s corpuscles), which are borne upon some part of the leaves, and which are continuously renewed during the life of the leaf. These ants act as a bodyguard to their home against the inroad of leaf-cutting ants collecting material for their fungus-gardens. These remarkable relationships have naturally led to the assumption that there is causal connexion between the ants and the
