Page:EB1911 - Volume 21.djvu/795

Rh the ordinary laws of pollination. Heterophylly is rather common among aquatic plants, and is well seen in several aquatic species of Ranunculus, many species of Potamogeton, Sagittaria sagittifolia, Scirpus lacustris, Castalia (Nymphaea) alba, Hippuris vulgaris, Callitriche spp., Sium latifolium.

Insectivorous species occur among aquatic plants; e.g. Utricularia spp., which are locally abundant in peaty waters, are insectivorous.

Xerophytes.—These plants have devices (a) for procuring water, (b) or for storing water, (c) or for limiting transpiration; and these adaptations are obviously related to the physically or physiologically dry habitats in which the plants live. Plants of physically dry habitats, such as deserts and sand dunes, have frequently long tap-roots which doubtless, in some cases, reach down to a subterranean water supply. The same plants have sometimes a superficial root system in addition, and are thus able to utilize immediately the water from rain showers and perhaps also from dew, as Volkens maintains. Root-hairs give an enlarged superficial area to the roots of plants, and thus are related to the procuring of water.

The stems of some xerophytes, e.g. Cactaceous and Crassulaceous plants, may be succulent, i.e. they have tissues in which water is stored. Some deserts, like those of Central America, are specially characterized by succulents; in other deserts, such as the Sahara, succulents are not a prominent feature. Other xerophytes again are spinous. “Switch plants,” such as Retama Retam and broom (Cytisus scoparius), have reduced leaves and some assimilating tissue in their stems; and stomata occur in grooves on the stem.

The transpiring surface of xerophytes is frequently reduced. The ordinary leaves may be small, absent, or spinous. In “cushion plants” the leaves are very small, very close together, and the low habit is protective against winds. The latter, of course, greatly increase transpiration. A “cushion plant” (Anabasis aretioides) of the north-western Sahara, frequently shows dead leaves on the exposed side whilst the plant is in full vigour on the sheltered side. The buds and leaves on the exposed side are probably killed by sand blasts. Many xerophytes are hairy or have sunken stomata which may be further protected by partial plugs of wax: the stomata are frequently in grooves: the leaves are frequently rolled—sometimes permanently so, whilst sometimes the leaves roll up only during unfavourable weather. These adaptations tend to lessen the amount of transpiration by protecting the stomata from the movements of the air. In species of Eucalyptus, the leaves are placed edge-wise to the incident rays of light and heat. The coriaceous leaves of “sclerophyllous plants” also, to some extent, are similarly protective. In such leaves, there are a well-marked cuticle, a thick epidermis a thick hypodermis at least on the upper side of the leaf, well-developed palisade tissue, and a poorly developed system of air-spaces. Such adaptations are well seen in the leaf of the holly (Ilex aquifolium). Warming, however, states that “Ilex aquifolium is undubitably a mesophyte” (1909: 135).

Halophytes, or plants which live in saline soils, have xerophytic adaptations. A considerable proportion of halophytes are succulents, i.e. their leaves and, to some extent, their stems have much water-storing tissue and few intercellular spaces. Some halophytes tend to lose their succulence when cultivated in a non-saline soil, and some non-halophytes tend to become succulent when cultivated in a salty soil; there is, it need scarcely be stated, little or no evidence that such characters are transmitted. British salt marshes furnish few instances of spiny plants, though such occur occasionally on the inland salt marshes of continental districts. Salsola Kali is British, and a hemi-halophyte at least; and it is rather spiny. Warming states that “the stomata of true, succulent, littoral halophytic herbs, in cases so far investigated, are not sunken” (1909: 221). It is possible, however, that the absence of sunken stomata, and the occurrence of some other halophytic features, are related merely to the succulent habit and not to halophytism, for succulent species often occur on non-saline soils. Similarly, the small amount of cuticular and of epidermal protection, and of lignification in succulent halophytes may also be related to the same circumstance. Forms of “stone cells” or “stereids” occur in some of the more suffruticose halophytes, as in Arthrocnemum glaucum. The interesting occurrence of certain halophytes and hemi-halophytes on sea-shores and also on mountains is probably to be explained by the past distribution of the species in question. At one time, such plants were probably of more general occurrence; now they have been extirpated in the intermediate localities, chiefly owing to the cultivation of the land in these places by man. In the west of Ireland and in the Faröes, where certain inland and lowland localities are still uncultivated, Plantago maritima and other halophytes occur in quantity and side by side with some “Alpine species,” such as Dryas octopetala.

The effect of common salt on the metabolism of plants is not understood. Lesage has shown that the height of certain plants is decreased by cultivation in a saline soil, and that the leaves of

plants under such conditions become smaller and more succulent He showed further, that the increase of common salt in the soil is correlated with a reduction in the number and size of the chloroplastids, and therefore in the amount of chlorophyll. On the other hand, some plants did not respond to the action of common salt, whilst others were killed. Warming (1909; 220) quotes Griffon (1898), to the effect that “the assimilatory activity is less in the halophytic form than in the ordinary form of the same species.” Schimper had previously maintained that the action of common salt in the cell-sap is detrimental as regards assimilation. Many marine Algae appear to be able to regulate their osmotic capacity to the surrounding medium; and T. G. Hill has shown that the root hairs of Salicornia possess this property. There has, however, been performed upon halophytes very little physiologically experimental work which commands general acceptance.

Bog Xerophytes live in the peaty soil of fens and moors which are physically wet, but which are said to be physiologically dry. Related to the physiological drought, such plants possess some xerophytic characters; and, related to the physical wetness, the plants possess the aeration channels which characterize many hydrophytes and hemi-hydrophytes. The occurrence of xerophytic characters in plants of this type has given rise to much difference of opinion. It is sometimes maintained, for example, by Schimper, that their xerophytic characters are related to the physiological dryness of the habitat: this, however, is denied by others who maintain (Clements, 1905: 127) that the xerophytism is due to the persistence of ancestral structures. It is possible, of course, that each explanation is correct in particular cases, as the views are by no means mutually exclusive. With regard to the occurrence of plants, such as Juncus effusus, which possess xerophytic characters and yet live in situations which are not ordinarily of marked physiological dryness, it should be remembered that such habitats are liable to occasional physical drought; and a plant must eventually succumb if it is not adapted to the extreme conditions of its habitat. The xerophytic characters being present, it is not surprising that many marsh plants, like Juncus effusus and Iris pseudacorus, are able to survive in dry situations, such as banks and even garden rockeries.

Tropophytes.—These plants are characterized by being xerophytic during the unfavourable season. For example, deciduous trees shed their leaves in winter: geophytes go through a period of dormancy by means of bulbs, rhizomes, or other underground organs with buds; whilst annuals and ephemerals similarly protect themselves by means of the seed habit. All such plants agree in reducing transpiration to zero during the unfavourable season, although few or no xerophytic characters may be demonstrable during the period favourable to growth.

Hygrophytes.—Living, as these plants do, under medium conditions as regards soil, moisture and climate, they exhibit no characters which are markedly xerophytic or hydrophytic. Hence, such plants are frequently termed mesophytes. Assimilation goes on during the whole year, except during periods of frost or when the plants are buried by snow. An interesting special case of hygrophytes is seen with regard to plants which live in the shade of forests. Such plants have been termed sciophytes. Their stomata are frequently not limited to the underside of the leaves, but may occur scattered all over the epidermal surface. The epidermal cells may contain chlorophyll. Strengthening tissue is feebly developed. Many sciophytes are herbaceous tropophytes, and are dormant for more than half the year, usually during late summer, autumn and early winter. It may be that this is a hereditary character (cf. “bog xerophytes”), or that the physical drought of summer is unfavourable to shade-loving plants. In this connexion, it is interesting that in the east of England with the lowest summer rainfall of this country, many common sciophytes are absent or rare in the woods, such, for example, as Melica uniflora, Allium ursinum, Lychnis dioica, Oxalis Acetosella, and Asperula odorata. However, the cause of the absence or presence of a given species from a given locality is a department of ecology which has been studied with little or no thoroughness.

Calcicole and Calcifuge Species.—Plants which invariably inhabit calcareous soils are sometimes termed calcicoles; calcifuge species are those which are found rarely or never on such soils. The effect of lime on plants is less understood even than the effect of common salt. Doubtless, the excess of any soluble mineral salt or salts interferes with the osmotic absorption of the roots; and although calcium carbonate is insoluble in pure water, it is slightly soluble in water containing carbon dioxide. In England, the following species are confined or almost confined to calcareous soils: Asplenium Ruta-muraria, Melica nutans, Carex digitata, Aceras anthropophora, Ophrys apifera, Thalictrum minus, Helianthemum Chamaecistus, Viola hirta, Linum perenne, Geranium lucidum, Hippocrepis comosa, Potentilla verna, Viburnum Lantana, Galium asperum ( = G. sylvestre), Asperula cynanchica, Senecio campestris. The following plants, in England, are calcifuge: Lastraea Oreopteris, Holcus mollis, Carex echinata, Spergula arvensis, Polygala serpyllacea, Cytisus