Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/484

 438

ANGIOSPERMS

features are found in many cycles of affinity amongst have a root, and there is always reserve-food stored m Dicotyledones and Monocotyledones. Typical hydrophytes some part of it. In essentials such a bud resembles a live submerged in water. Their general physiognomy is seed. A relation between such vegetative distribution that of delicate flexible herbs which rapidly shrivel on buds and production of flower is usually marked. Where exposure to the air. In their environment their waterthere is free formation of buds there is little flower and is not a first charge upon the organization, because commonly no seed, and the converse is also the case. supply diffusion can take place at every point, but the gaseous Viviparous plants are an illustration of substitution of supply, owing to the slow diffusion of air in water, and vegetative buds for flower. II. Aerophytes—Th.e oecological category of Aerophytes the supply of radiant energy, owing to the feeble penetration of light, are matters of first moment. Further, the is a much smaller one than that of Geophytes, and, un- density of the medium reduces the need of intrinsic like the latter, they are not affected by edaphic environ- support. To those conditions hydrophytes are adapted ment. An aerophyte is an autotrophic plant growing upon first of all by an extension of the shoot-surface, secured another, to which it is commonly fixed by its roots. The elongation of the leaves into a riband form in moving term epiphyte, commonly given to them, is so frequently by or by their division into filiform segments in still used to designate the position of saprophytes and parasites water, water; at the same time the system of air-canals is upon host-plants, that it is better avoided as the name of greatly developed. Then the chlorophyll apparatus lies in the oecological category we are dealing with. Aerophytes are found amongst both Dicotyledones and Monocotyle- the peripheral layers of the organism, palisade-tissue being dones and in many families. They may be herbs, shrubs, absent, and the strengthening tissue is always reduced in or trees. They are derived from geophytes, and their mode amount, and so forth. In the absence of transpiration there of life, while it enables them to reach better conditions is a reduction or, it may be, a suppression of the rootof light, makes them dependent wholly upon epedaphic system, and it is often only developed as an anchoring environment, and their distribution over the world is organ. No stomata are formed upon the shoot, and the markedly contingent upon a supply of atmospheric mois- cuticle is everywhere only feebly formed. The carrying ture. Their home is the damp woods of the tropics and system too is greatly simplified, both by reduction of the of the warm temperate regions. The conditions of life of number of vasa and of their area, and it is concentrated an aerophyte make its supply of water somewhat pre- as a central strand in the axis, just as it is in ordinary carious ; it has therefore to provide for the protection of roots, and thus is favourably placed in relation to the what it secures, and for the rapid absorption of any coming pulling strains to which the shoot is subjected under the within reach. For the former of these purposes it most currents of water. The growing points are frequently incommonly has some form of water-storage which gives it vested by mucilage, which acts as a protection to them. succulence in leaf or stem; the cuticle is thick and the Many of these features of hydrophytes are observable in stomata are sunk, and a coating of hairs is found occasion- hygrophilous geophytes, and there are all stages of tranally though rarely. For the latter purpose the roots, sition between the two types. Thus when in hydrophytes which are frequently copious, often have a velamen; ex- the flowers are brought above the surface of the water, one posed to light they form chlorophyll and become accessory or more flat floating leaves may be developed to support organs of carbon-dioxide-assimilation, or they may take on the plant at the surface, or the floating leaves may prethe whole duty of this, the leaves being then suppressed. dominate. Floating plants also occur in which the leaves Sometimes in rootless forms absorptive hairs enable the are variously modified to act as floats. In all cases where plant to take water by its whole surface. The seed of an parts of the shoot reach the air they develop more or less aerophyte usually germinates upon the supporting plant, geophytic features. Amongst the most remarkable of and it is rare for a geophyte subsequently to become an hydrophytes are the Podostemacese — tropical dicotyleaerophyte; on the other hand several cases are known donous plants of cascades and rapidly flowing water. of a juvenile aerophyte becoming a geophyte when older, They have the habit of Bryophytes. The root forms a and smothering its nurse as it descends to the soil. These branched dorsiventral chlorophyllous thallus-like structure, plants are therefore only hemiaerophytes. In some aero- attached by hairs and special branches (haptera) to rocks, philous Bromeliads the rosette of leaves forms a cup in and it is the chief structure for the assimilation of carbon which water collects; into this debris both organic and dioxide. The shoots, which are also commonly dorsiveninorganic falls, and produces a nutrient solution in which tral, arise as outgrowths from it and bear small, usually hydrophytes like Utricularia may live. The Bromeliad divided, leaves. Marine hydrophytes which are subject to absorbs this fluid by special hairs, and is therefore a exposure between tides have often some of the xerophilous partial saprophyte; its roots are only anchoring organs. features of halophytes. In their flower-structure typical Such plants are called Tank Aerophytes. A somewhat hydrophytes show great simplification where the flowers similar condition, differing only in soil being collected are persistently submerged. They are frequently uniinstead of water, is seen in Nest Aerophytes. Here some sexual and the flower-envelopes are reduced. For subroots or leaves form, with the stem of the supporting plant, aqueous pollination the pollen is often vermiform of about a receptacle in which debris accumulates and makes a soil the same specific gravity as the water, whilst the styles into which feeding roots from the aerophyte pass; in are long and filiform so as to increase the chances of the this case there is an approach to the geophytes. Many act being accomplished. Sometimes pollination takes aerophytes have brilliant flowers highly specialized for place on the surface of the water, and then in various ways entomophilous pollination. The seeds are small and light, the pollen is brought to the surface, where it comes in or have flight arils or a succulent fruit-envelope, and contact with the stigmas. Often the flowers are raised in can readily enter a crack of tree-bark. Dissemination is the air above the water, and then they exhibit the features anemophilous or zooidiophilous, sometimes also the rain of anemophily or zooidiophily. Many hydrophytes have may be the agent. Vegetative propagation hardly exists, embryos of peculiar form and remarkable germination. but portions of the shoot of Tillandsia usneoides are said Amongst Monocotyledones most hydrophytes are exalbuminous and the embryos are macropodous through storage to be carried by wind and birds. III. Hydrophytes.—Plants of the oecological category of of food-material. Vegetative propagation by the detachHydrophytes dwell in a more uniform environment than ment of portions of the shoots or of definite buds is not is the lot of Geophytes or Aerophytes, and hydrophilous uncommon amongst hydrophytes, and these, readily