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PROPAGATION, DEVELOPMENT, AND CULTURE.
N aturally Terns are propagated by means of the spores,
of which mention has been already made. These spores
are somewhat analogous to seeds, being like them endowed
with that mystery—the vital germ; and, when placed under
fitting conditions, they become developed into young plants;
but they differ from seeds in some important particulars.
All true seeds have a determinate structure; they have
an embryo, with special organs, namely, the plumule, or
germ of the ascending axis, the origin of the stem, and
the radicle, or germ of the descending axis, the origin of
the root. When a seed is planted, in whatever position it
may chance to have been deposited in the soil, the young
root or radicle strikes downwards, and the young stem or
plumule grows upwards.
The Tern spores have none of these determinate parts,
but are, as it were, homogeneous atoms; and when placed
under circumstances which induce germination, that part
which lies downwards produces the root, and that part
PROPAGATION, DEVELOPMENT, AND CULTURE. 21
which lies upwards produces the rudimentary stem. The
spores are very minute vesicles of various shapes, but
mostly roundish, and are often beautifully ornamented with
markings on the exterior. They consist merely of a small
vesicle of cellular tissue, and as they grow this vesicle
becomes divided into others, which again multiply and
enlarge, until they form a minute green leaf-like patch,
roundish but irregular in outline, unilateral, and often, if not
always, two-lobed, forming a primordial scale or leaf; this
by degrees thickens at a central point on the side, which
henceforth becomes the axis of development, and from this
point a small leaf or frond is produced on the upper surface
where the tissue is acted on by light. This leaf is usually
very different in aspect as well as size from the mature
fronds, and is succeeded by other fronds, which acquire by
degrees the characteristic features peculiar to their species.
In some annual Terns the mature character is soon attained,
hut in others two or more years of growth is required
before they reach maturity; they, however, soon
begin to assume something of their peculiar appearance,
so that by the time three or four of these young fronds
are produced, sometimes even earlier, a practised eye can
recognize the species.