614 
THE PRINCIPLES OF BOTANY. 
By Professor James Buckman, F.L.S., F.G.S., &c. 
( Continued from p. 549.) 
Class II. — Acrogens. 
W itii this class, as was observed by Professor Lindley, a 
great advance in structure is accomplished. The simplicity 
which is so remarkable in Thallogens is exchanged f or a com- 
plicated apparatus of many kinds. All the species have 
stomates or breathing-pores on their surface. In the great 
majority there are a distinct stem and leaves, the latter of which 
are always arranged in perfect symmetry, and in those species 
which approach Thallogens (as crystal- worts which stand 
close upon Lichens) the thallus has all the texture of leaves, 
although a separate stem is refused to them. There is, how- 
ever, no trace of flowers properly so called, and yet in the 
involucre of many liverworts and in the spore-cases of mosses 
an arrangement of leaves occurs which appears to be the 
forerunner of the flowers of more perfect plants. Sexes, 
however, are wholly missing, that is to say, nothing can be 
found that resembles the anthers and pistil of flowering 
plants except in some vague external circumstance. There 
is no evidence to show that any order of Acrogens possesses 
organs which require to be fertilised the one by the other, 
in order to effect the generation of seeds. Hence those re- 
productive bodies of Acrogens which are analogous to seeds 
are called spores, as they are in more lowly organized plants, 
but in the spore-cases of mosses, the discs of Equisetums, and 
the sori of ferns we recognise a more highly developed or- 
ganization than is to be found in the thalliform nuclei of 
Lichens or Algae or the gills of Agarics or Boleti. 
As with the thallogens so with the present class, we shall 
first lay down their more general characters in the form of 
alliances, and subsequently refer to some of their principal 
orders. 
The following, then, are the 
Alliances of Acrogens. 
Muscales. — Cellular (or vascular) spore-cases immersed or calyptrate 
( i . e., either plunged in the substance of the frond, or enclosed 
within a hood, having the same relation to the spores as an 
involucre to a seed-vessel). 
