e is no stem, but only a connected series
of green disk-like expansions, while others have a distinct stem with
leaf-like outgrowths.

Two genera of aquatic plants (_Chara_ and _Nitella_) constitute another
group of plants called _Characeae_. These will be hereafter referred to
both on account of peculiarities in their structure and on account of a
peculiar motion of protoplasm which is easily to be seen[22] in them.

Mosses (_Musci_) are familiar objects to every one in this country, and
allied to them are the so-called "club-mosses" or _Lycopods_, which form
a sort of green sward in so many parts of the warmer regions of the
earth. To one of the lycopods, called _Selaginella_, reference will
hereafter be made in connexion with its very instructive reproductive
process.

Certain humble plants, in some of which the foliage leaves present a
superficial resemblance to those of a four-leaved clover, are popularly
called pepperworts; by botanists, _Rhizocarpeae_ or _Marsiliaceae_. They
are creeping or floating stemless plants which inhabit ditches or
inundated places. They are scattered over both the Old and New Worlds,
but are chiefly found in temperate latitudes.

The horse-tails (_Equisetaceae_) are also found in most parts of the
world, though wanting in Australia and New Zealand. They inhabit wet and
sandy places, and sometimes are of a considerable size even in the
present day, but in ancient geological periods they attained the
proportions of trees.

This group leads us on to their allies the ferns which form a very large
natural group _Filices_ or _Pteridophytes_--a group now familiar to
every one interested in plants. Common as ferns are in our own country,
they are far more abundant and attain to a much greater size in southern
latitudes--notably in New Zealand and various Pacific islands.

All the plants hitherto enumerated, from the protococcus to the
tree-ferns inclusive, together form what is commonly regarded as one
great primary division or "sub-kingdom" of vegetals called CRYPTOGAMIA.
In no plant belonging to this sub-kingdom--in no single cryptogam--is
any flower ever developed. These form the great group which is often
spoken of as "flowerless plants."

The other primary division of vegetable organisms consists of all plants
with flowers, and is termed PHANEROGAMIA, and is subdivided into two
sections,[23] very unequally numerous. To the first section of
phanerogams--a section containing comparati