3. They are formed only from rudimentary carpels, in which the stem
takes no part.
4. The number of carpels in each flower varies from one to many, most
frequently three, of which the central one remains sterile.
5. Each carpel bears only one ovule. The flower which is formed of only
one carpel appears to consist of only one ovule.
6. The ovule has in Taxaceæ either a double integument (Podocarpeæ,
Taxeæ), of which the external is the “aril,” or, as in the Cycadeæ, a
single one, which is homologous with the two united together.
7. The external integument in the Pinoideæ is expanded to form a
leaf-like structure--the ovuliferous scale--and bears on its dorsal
side the ovules, which are thus only provided with one, and that the
inner, integument.
This later interpretation of the female cones in the Coniferæ is
more probably correct than the older ones; that, however, which
appeared in the former issues of this book, may also be stated.
It was to the effect that each catkin-like female cone is in
reality a single flower; the cone-scales in the Cupressaceæ
were single leaves, namely carpels, which bore the ovules on
the side which is turned upwards; the division into two parts
which makes its appearance in the other orders, and becomes
most prominent in the Abietaceæ, was compared with the division
of a leaf into a barren and a fertile portion, which is found
especially in Ophioglossaceæ and Marsiliaceæ, or with the ligule
in _Isoëtes_.
POLLINATION is accomplished by means of the wind. At the period
of pollination the leaves are always so widely separated from one
another, that the ovules can catch the pollen-grains carried to
them by the wind; this is often effected by the mucilaginous drops
which appear at the micropyle, and by the evaporation of which the
pollen-grains are brought in contact with the nucellus. The entire cone
grows considerably as soon as fertilisation has taken place, and the
cone-scales in Pinoideæ close together so that the seeds while maturing
are enclosed, and it is not until the seeds are ready for distribution
that the cone-scales again become separated. In the Pinoideæ, the fully
developed ovuliferous scales are hard and woody; and in this condition
the collection of female flowers is termed a _cone_. In the Taxoideæ,
true cones are the exception. 2–15 cotyledons are present, arranged in
a whorl.
The characteristic feature of this class is the abundance of _resin_,
which is to be found in isolated cells (especially in the cortex),
partly in intercellular glands or passages (both in the cortex and
wood). _Taxus_ is the only genus which has no resin.
There are about 350 species, mostly from the Northern Temperate
zone (especially North America and Siberia), where they grow
gregariously and form the most northern forests. The Juniper,
Scotch Fir, and Yew are natives of Great Britain.