52 ACROGENS. 



threads iu tlieir cells. This has been long known to exist in the bodies 

 called elaters among Liverworts, and traces of it have been recognised in the 

 leaves of certain mosses, such as Sphagnum. 



" So far as I am aware," says Schleiden, '* the occurrence of a spiral 

 formation has been observed in the reproductive organs of Hepaticae only in 

 the elaters or fruit-valves. But it is not less strikingly developed in the 

 organs of vegetation in Marchantiace?e. The parenchyma of the leaf of 

 Marchantia polymorpha and Fegatella conica consists almost entirely of 

 cells whose partitions appear distinctly porous, or (especially in M. poly- 

 morpha) beautifuU}" thickened with net-work. This thickening of the 

 partitions of the cell takes place to so great a degree in the older parts 

 and in the proximity of the midrib, that by transverse sections the pore- 

 channels may be plainly recognised. Amongst mosses, the true Dicrana, 

 for example D. Schraderi, spurium, (kc, are distinguished by the cells of 

 the leaf having very thick sides, and their partitions evidently pierced by 

 very wide, or funnel-shaped pore-channels, just as happens in the epidermis 

 of many phanerogamous plants ; and still more conspicuously do these spiral 

 and porous formations display themselves in Sphagnese, and in the nearly 

 related group of Leucophaneae established by Hampe." — {Ami. Nat. 

 Hist. V. 73.) The same tendency is still more remarkably apparent in a 

 curious formation of loose short spiral threads generated in the cells of the 

 bodies called Antheridia, and elsewhere ; which, because of an apparently 

 spontaneous motion when they are floating in water, have been thought to 

 be animalcules of the genera Spirillum or Vibrio. 



In general, Acrogens are plants of very small stature. But in Ferns they 

 occasionally acquire the size of trees ; always however growing with a simple 

 stem in such cases, unless when their grovrth is interrupted by accident. 

 If they branch naturally, they do so in a forking manner. Their stem, 

 instead of increasing by the deposition of matter originating in the leaves, 

 appears to be a mere extension of one common vegetating point, which 

 becomes cylindrical and long, when it is capable of being acted upon by the 

 influence of Hght. It may be regarded indeed as a mere combination of the 

 bases of leaves, gradually evolved one from the bosom of the other. 



The orders of Acrogens seem to resolve themselves into three Alliances, 

 of which the lowest in organization in some respects is the highest in 

 others. This which is named the Muscal, inasmuch as it includes the true 

 Mosses, has no spiral vessels, no veins to its leaves, and its species are of 

 diminutive size ; but it has reproductive organs of two very distinct kinds, and 

 its spore-cases are usually elaborately pro\dded with elaters at least, and often 

 with a complicated arrangement of rudimentary leaves. Tlie two otliors 

 have a far larger stature, are abundantly furnished with scalariform or true 

 spiral vessels in their stem, but their reproductive organs are of the most 

 simple kind, and never assume different forms in the same individual. 

 The one called the Lycopodal AUiance has scaly leaves and pulverulent 

 spores, always of two sorts, contained in cases which usually open by 

 definite valves ; the other, called the Filical Alliance, has thin expanded 

 veiny leaves and granular spores of only one kind enclosed in cases which 

 burst irregularly. 



The affinities of Acrogens are well ascertained. Riccia and its neigh- 

 bours are closely allied to Lichens. Horsetails may be looked upon as an 

 approach towards the structure of Ephedra among Gnetacese, or of Casuarina 

 in Galeworts. The Clubmosses evidently approach Coniferous Gymnogens 

 in their small scale-like imbricated leaves and coniferous fructification. 



