In this class THE EMBRYO has 2 seed-leaves, a rule from which there are
few exceptions (_e.g. Ficaria_, _Cyclamen_, _Pinguicula_, certain
species of _Corydalis_, with only 1; and a few, mostly parasitic
forms, _e.g. Monotropa_, _Orobanche_, _Pyrola_, entirely without
cotyledons). On germination the cotyledons nearly always raise
themselves above the ground as green, assimilating leaves and are then
termed aerial or epigean, in contradistinction to the underground or
hypogean which are always buried. The structure of the seed varies
(endospermous or exendospermous); the embryo may be straight or curved.
In many instances the primary root grows as a vigorous tap-root,
with weaker branches arising acropetally (in annuals, biennials,
many perennials, especially woody plants); but in a large number of
herbaceous perennials, which have rhizomes, the root behaves very much
as in the Monocotyledons. The roots generally increase in thickness by
means of a cambium.
THE STEM, when seen in transverse section, has its vascular bundles
arranged in a ring; in reality, however, they form a kind of
cylindrical network in the stem; the bundles are open, and thickening
takes place by means of a cambium; annual rings are formed in the
perennial stems. There is a rich and very varied form of _branching_.
The two first leaves of a shoot (fore-leaves) are placed nearly always
to the right and to the left; the same arrangement is found in the two
first leaves developed on the flower-stalk, and these are, as a rule,
the only two; they are found below the calyx and are usually termed the
“_bracteoles_.” It has become customary to indicate the bracteoles by
the letters α and β, according to their sequence of growth, and in that
sense these letters will be employed in the following diagrams.
THE ARRANGEMENT OF THE LEAVES varies very much; there is also a great
variety of shapes in the leaves and their venation, but the linear
leaves, with parallel venation, so frequent in the Monocotyledons,
are seldom met with, as also the large sheaths (though the sheath is
well developed in the Umbelliferous plants); stipules occur much more
frequently.
THE FLOWER is most commonly cyclic, but acyclic or hemicyclic forms
also occur. The type which may be taken as a basis consists in the
majority of instances, as in the Monocotyledons, of 5 whorls, of which
the 4 outer ones (calyx, corolla, and the 2 whorls of stamens) are
most frequently 4 or 5 in number and placed in regular alternation,
whilst the innermost one (the carpels) has generally fewer members,
probably on account of space (Figs. 360, 361, 421, 429, 487, etc.).
Trimerous (Figs. 384, 387, etc.) flowers, or those in which the members
of the flower are in threes or a multiple of three, also occur, as well
as dimerous flowers; other numbers are rare. It is of the greatest
importance in connection with the relative position of the members of
the flower to the axis and bract (orientation), whether the bracteoles
are typically present (even though they may not be developed), or
are typically absent. If there are 2 bracteoles present, then their
position in a pentamerous flower is often as follows: the first sepal
turns obliquely forward, the second is posterior and median, the
third obliquely forward, the fourth and fifth obliquely backward;
quincuncial æstivation is often found in these buds (Figs. 360, 429,
471, 475, 584). The first and third leaves, in the following chapters,
are most frequently alluded to as the “anterior,” the fourth and
fifth as the “lateral” leaves. The _reversed_ arrangement, with the
median sepal in the front, occurs for instance in _Papilionaceæ_ (Fig.
511), _Lobeliaceæ_ (Fig. 594), _Rhodoracecæ_. If any bracteoles are
present below a tetramerous flower, the relation is generally that
2 sepals (the first ones) stand in the median plane, the two next
ones transversely (Fig. 393), and the corolla then adopts a diagonal
position (Fig. 397); but a diagonal position of the calyx generally
shows that the flower is not, strictly speaking, tetramerous, as in
_Plantago_ (Fig. 567), _Veronica_ (Fig. 559 _C_) and others.
If the bracteoles are _not_ typically present, then the position of the
sepals is changed accordingly, and the two outer sepals endeavour to
assume the position which the bracteoles would otherwise have occupied,
_e.g._ in _Primula_ (Fig. 547). Other positions are also found when the
number of bracteoles is more or less than two.
The leaves which follow the sepals occupy definite positions with
regard to them, which we may consider later. An arrangement must,
however, be mentioned here; when the flower is “_diplostemonous_” that
is, has two whorls of stamens (thus, Sn, Pn, An + n), these may be
arranged in two ways. _Either_ the first-formed whorl of stamens, which
are termed the “calyx-stamens,” stands directly in front of the sepals
(that is “episepalous”), and is the _outermost_ whorl, and in this case
a regular alternation takes place between sepals, petals and the two
whorls of stamens, which is also continued into the carpels if their
number is the same as that of the other whorls: the carpels are then
placed opposite the sepals (Fig. 278) and the flower is _isomerous_
and Gn should be added to the formula above. _Or_, the calyx-stamens
form the _innermost_ whorl, and the corolla-stamens, which are
subsequently formed (“epipetalous” stamens), stand _outside_ these
(Figs. 360, 429); if the number of carpels is the same as that of the
preceding whorls, they are often placed _right in front_ of the petals
and the corolla-stamens. The first-mentioned arrangement is termed
_Diplostemonous_, and the second _Obdiplostemonous_. ~Both arrangements
may be found in one and the same order, _e.g._ Caryophyllaceæ. The
size and relation of the members of the flowers, and also the contact
with other members in the early stages of their development, play an
important part in determining the arrangement.~
The great number of structural arrangements found in this enormously
large class, may, as is the case in the Monocotyledons, be further
varied by _suppression and division_ of certain leaves (especially the
stamens). Instances of this will occur in the following (Figs. 559,
568.--426, 441, 445, etc.).
The Dicotyledons were formerly divided into 3 sub-classes: Apetalæ
(those without corolla), Sympetalæ or Gamopetalæ (those with the petals
united), and Choripetalæ or Polypetalæ (the petals not united). This
division has now been abandoned because it has been proved that the
Apetalæ were merely reduced or incomplete forms of the Choripetalæ, and
they have therefore been distributed among the various families of the
latter sub-class.
With regard to the Sympetalæ (or Gamopetalæ) it may be stated that
they form to a very great extent a closely connected and natural
group, having in common not only the character that the corolla is
gamopetalous and the stamens united with it (this being also found in
the Choripetalæ), but also a great many others (such as persistent
calyx, cyclic flowers with the formula S5, P5, A5 and as a rule G2,
the two carpels being united to form the ovary; seeds with a thick
integument and a very small nucellus). They are therefore considered
as an independent sub-class, and must be placed at the close of the
system of classification as the forms which presumably have arisen the
latest. In the future systems of classification this arrangement will
very probably be changed, and the first families of the Sympetalæ, the
Bicornes and others will for instance be to a certain extent united
with the families or orders of the Choripetalæ. The Sympetalæ may
certainly be considered as the youngest types, the strongly pronounced
metamorphosis supporting this theory, as also the formation of the
integument of the ovule, the one thick integument being undoubtedly
derived from the coalescence of two--a holochlamydeous ovule, etc.
The Apetalæ and Choripetalæ are united into one sub-class. The leaves
of the perianth in this case are, as a rule, free from each other, the
structure of the flowers presents many differences, and the ovules have
as a rule 2 integuments and a large nucellus. Considerable uncertainty
still prevails regarding the arrangement and the relationship of the
individual families of the Choripetalæ, and some of the following
families are hardly quite natural; but the best arrangement arrived at
so far has been adopted here.
At the beginning of the book a review of the orders of the Dicotyledons
will be found.