THE STRUCTURE OF MOSSES. 
11 
nation of generations. The most marked distinction between 
alternate generations is that between the sexual and the 
asexual. In the very lowest forms of algae and fungi there is 
no sexual generation and no alternation. All the cells are 
similar, and propagate by simple fission without fertilisation. 
In the higher forms there is a sexual generation, soijietimes 
regularly alternating with an asexual one, sometimes coming 
in at more distant intervals, and sometimes there is a third 
generation different from either. But mosses, in common 
with all the vascular cryptogams above them, have an 
unchangeable succession of sexual and asexual generations. 
The germinating spore produces a branching plant, with 
stem and leaves, a true cormophyte— Kopp.os <pvrov —a plant with 
a stem. This is the sexual generation, bearing the antheridia 
which fertilise the arcliegonia. But the fertilised arcliegonium 
does not develop into a seed or a true fruit. It gives rise 
to a slender thread-like branch, which afterwards enlarges 
at the end into a thallus, on the surface of which arise 
spores requiring no fertilisation. This is the asexual gene¬ 
ration. The thallus in mosses takes the form of a capsule, 
and is commonly called a fruit; but morphologically it is 
the same as the spore-bearing thallus of the tliallophytes 
—the fungi and algae—and not homologous with the fruit of 
phoenogams. 
Thus in Mosses the cormophyte generation is sexual and 
the tliallopliyte asexual, while in ferns the cormophyte or 
stem and leaf form is asexual, the spore bearing frond ; and 
the thallophyte—the pro-tliallus, is the sexual generation 
bearing antheridia and arcliegonia. 
When our present systems of classification were founded, 
these alternate generations were but little understood, and 
were not regarded. But they seem to be so fundamental to 
the different modes of growth and development that they will 
probably take a conspicuous place in the classifications of the 
future. 
Now let us trace the various stages of growth in a moss. 
When a spore germinates, it produces first a branching 
septate thread on the surface of the ground, the cells of 
which contain green chlorophyll. This is called the 
■protoncma. From different parts of this thread leaf-buds 
arise, which grow at the points into stems producing leaves 
as they lengthen. 
The leaves are sometimes disticliou, that is in two 
opposite rows, but more often alternate and in spirals of 
different formulae, which cause the leaves to stand in three, 
five, or eight rows. 
