88 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



separated from the main stem. In true hydrophytes flowers are not 

 nearly so abundant or conspicuous as in land plants. 



Pondweed societies include numerous species which float freely upon 

 or below the surface of the water (e.g. many Alga?, Duckweeds, Bladder- 

 wort, Water- violet, Frogbit, &c), but the majority are anchored to the 

 bottom (Alga>, Mosses, Pondweeds, Water-lilies, &c). 



Swamp societies consist of plants growing in shallow water, or in 

 very wet soil, with stems and leaves rising well above the surface, and 

 therefore agreeing more nearly with those of mesophytes or ordinary 

 terrestrial plants in their structural characters. A distinct zonal arrange- 

 ment can frequently be perceived in the grouping of such plants. Thus 

 the edge of the fringe of vegetation bordering a lake or sluggish stream is 

 often composed of clumps of Arrowhead, Water-plantain, &c. ; then comes 

 a zone occupied by tall, slender, narrow-leaved plants, such as Flowering 

 Rush, Reed-mace, Bulrush, Burr-reeds, Sedges, and Reed-grasses ; and 

 behind these, again, are masses of Purple Loose-strife, Hemp-agrimony, 

 Willow-herb, Meadowsweet, and Angelica, with Willows, Alders, and 

 other moisture-loving shrubs and trees forming a background to the 

 whole. (Figs. 7, 8, and 9.) 



Here also belong peat bogs and fens, largely composed of Sphagnum 

 Moss, with coarse Grasses, Sedges, Cotton-grass, Sundews and Butter- 

 worts, Bog-orchids, &c. 



Hydrophytes pass imperceptibly into mesophytes, and indeed indi- 

 viduals of the same species may grow under both sets of conditions, and 

 then exhibit corresponding differences in structure and habit. Polygonum 

 amphibium is a good example of this. When growing in deep water it 

 has very long lax stems and smooth broad leaves which float on the 

 surface. The terrestrial form, on the other hand, possesses erect stout 

 steins, bearing numerous lanceolate and somewhat hairy leaves. 



Xerophytes. — These are adapted to resist drought and attain their 

 best development in arid, sandy, or rocky situations. The drought may 

 occur at irregular intervals or may be periodic, as in regions where wet 

 and dry seasons regularly alternate ; or it may be constant, as in certain 

 almost rainless desert regions. It is obvious that in such areas the 

 extremely limited water supply is accompanied by a tendency to increased 

 transpiration — conditions which are directly opposed to each other. 

 Hence these plants must develop contrivances for collecting and storing 

 water and for diminishing transpiration. 



Many xerophytes develop a special water-storage tissue — the aqueous 

 tissue — in the leaves, generally in the form of a layer of large colourless 

 cells between the upper epidermis and the palisade-tissue ; and an 

 extension of the storage function to all the leaf parenchyma produces the 

 thick fleshy leaves of succulent plants, such as the Crassulacece, Aloes, 

 Agave, &&, in which the leaves are usually very numerous and closely 

 packed together. Often the stem becomes stout and succulent, and the 

 leaves are then generally greatly reduced or even absent, or else modified 

 into spines or thorns : e.g. Salicornia, some Euphorbias, Cacti, &c. 

 These succulent plants not only store water in considerable quantity, but 

 they have a very great power of retaining it, and hence are able to survive 

 long periods of drought. (See fig. 10.) 



