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By Link they have been called Paraphyllia, and defined as "foliaceous parts, in structure like the leaves, and developed before those organs."
When they are membranous, and surround the stem like a sheath, cohering by their anterior margins, as in Polygonum (fig. 61. a), they have been termed ochrea by Willdenow. Of this the fibrous sheath at the base of the leaves of Palms, called reticulum by some, may possibly be a modification. In pinnated leaves there is often a pair of stipules at the base of each leaflet, as well as two at the base of the common petiole : stipules, under such circumstances, are called stipels.
What stipules really are is not well made out. De Candolle seems, from some expressions in his Organographie, to suspect their analogy with leaves; while, in other places in the same work, it may be collected that he rather considers. them special organs. I am clearly of opinion that, notwithstanding the difference in their appearance, they are really accessory leaves: first, because they are occasionally transformed, in Rosa bracteata, into pinnated leaves; secondly, because they are often undistinguishable from leaves, of which they obviously perform all the functions, as in Lathyrus, Lotus, and many other Fabacea: and, finally, because there are cases in which buds develope in their axils, as in Salix, a property peculiar to leaves and their modifications. De Candolle, in suggesting, after Seringe, that the tendrils of Cucurbitaceæ are modified stipules, assigns the latter a tendency to a transformation exclusively confined either to the midrib of a leaf, or to a branch; and they cannot be the latter. It is, however, more probable that the tendril in this order is an accessory bud, a little out of its place, as the Bravais' have suggested. (Ann. Sc., n. s., VIII. 20.)
It is sometimes difficult to distinguish from true stipules certain membranous expansions, or ciliæ, or glandular appendages of the margin of the base of the petiole, such as are found in Ranunculaceæ, Apocynaceæ, Apiaceæ, and many other plants. In these cases the real nature of the parts is only to be collected from analogy, and by comparing them with the same part differently modified in neighbouring species.
Link regards the scales of leafbuds (called by him tegmenta) as a kind of stipule, and such they, no doubt, sometimes are, as in Liriodendron; but then he unites with them the primordial ramentaceous leaves of Pinus, which have no analogy with stipules.
De Candolle remarks, that no Monocotyledonous plants have stipules; but they certainly exist, at least in Naiadaceæ and Araceæ. It is also said that they do not occur in the embryo; but then there are some exceptions to this statement, as well as to Miquel's remark, that they never occur upon radical leaves, e. g. Strawberry.
Turpin considers them of two kinds.
1. Distinct, but rudimentary, leaves, when they originate from the stem itself, as in Cinchonaceæ, &c.
2. Leaflets of a pinnated leaf, when they adhere to the leafstalk, as in Roses, &c.
The ligula of grasses, at the apex of their sheathing petiole, a membranous appendage, which some have considered stipulary, should rather be considered an expansion analogous to the corona of some Silenaceous plants.
It has been already noted, that when stipules surround the stem of a plant they become an ochrea; in this case their anterior and posterior margins are united by cohesion; a property which they possess in common with all modifications of leaves, and of which different instances may be pointed out in Magnoliacea, where the back margins only cohere, in certain Cinchonaceæ, in which the anterior margins of the stipules of opposite leaves are united, and in many other plants.
All the parts hitherto made the subject of inquiry are called organs of vegetation; their duty being exclusively to perform the nutritive parts of the vegetable economy. Those which are about to be mentioned are called organs of fructification; their office being to reproduce the species by a process in some respects analogous to that which takes place in the animal kingdom. The latter are, however, all modifications of the former, as will hereafter be seen, and as the subject of this division is in itself a kind of proof; bracts not being exactly either organs of vegetation or reproduction, but between the two.
Botanists call Bracts either the leaf from the axil of which a flower is developed, such as we find in Veronica agrestis; or else all those leaves which are found upon the inflorescence, and are situated between the true leaves and the calyx. There are, in reality, no exact limits between bracts and common leaves; but in general the former may be known by their situation immediately below the calyx, by their smaller size, difference of outline, colour, and other marks. They are often entire, however much the leaves may be divided; frequently scariose, either wholly or in part; sometimes deciduous before the flowers expand; but rarely very much dilated, as in Origanum Dictamnus, and a few other plants. It is often more difficult to distinguish bracts from the sepals of a polyphyllous calyx than even from the leaves
of the stem. In fact, there is in many cases no other mode than ascertaining the usual number of sepals in other plants of the same natural order, and considering every leaf-like appendage on the outside of the usual number of sepals as a bract. In Camellia, for example, if it were not known that the normal number of sepals of kindred genera is five, it would be impossible to determine the number of its sepals. When the bracts are very small, they are called bractlets; or, if they are of different sizes upon the same inflorescence, the smallest receive that name. It rarely occurs that an inflorescence is destitute of bracts. In Cruciferæ this is a general character, and is observed by Link to indicate an extremely irregular structure. When bracts do not immediately support a flower or its stalk, they are called empty (vacua). As a general rule, it is to be understood, that whatever intervenes between the true leaves and the calyx, whatever be their form, colour, size, or other peculiarity, comes within the meaning of the term.
Under particular circumstances bracts have received the following peculiar names :
When they are empty, and terminate the inflorescence, they form a coma, as in Salvia Horminum. In this case they are generally enlarged and coloured.
If they are verticillate, and surround several flowers, they constitute an involucre. In Apiaceous plants, the bracts which surround the general umbel are called an universal involucre ; and those which surround the umbellules a partial involucre, or involucellum. In Compositæ, the involucre often consists of several rows of imbricated bracts, and has received a variety of names, for none of which there appears to be occasion. Linnæus called it calyx communis, Necker perigynandra communis, Richard periphoranthium, Cassini periclinium. There is often found at the base of the involucre of Compositæ an exterior rank of bracts, which Linnæus called calyculus; and such involucres as were so circumstanced calyx calyculatus. Cassini restricts the term involucre to this; but it seems most convenient to call these exterior bracts bractlets, and to say that an involucre in which they are present is basi bracteolatum, bracteolate at the base.
Another form of the involucre is the cupula (fig. 66.). It consists of bracts not much developed till after flowering, when they cohere by their bases, and form a kind of cup. In the Oak the cupula is woody, entire, and scaly, with indurated bracts: in the Beech it forms a sort of coriaceous, valvular, spurious pericarp: in the Hazel Nut (fig. 65.) it is foliaceous and lacerated.*
In Euphorbia the involucre is composed of two whorls of bracts, consolidated into a cup, and assumes altogether the appearance of a calyx, for which it was for a long time mistaken.
The name squama or scale is usually applied to the bracts of the catkin; it is also occasionally used to indicate any kind of bract which has a scaly appearance.
The bracts which are stationed upon the receptacle of Compositæ, between the florets, have generally a membranous texture and no colour, and are called paleæ, Englished by some botanists chaff of the receptacle. The French call this sort of bract paillette, Cassini squamelles.
In Palms and Araceae there are seated, at the base of the spadix, large coloured bracts, in which the spadix, during æstivation, is wholly enwrapped, and which may perhaps perform in those plants the office of corolla. This is called the spathe (fig. 83.). Link considers it a modification of the petiole. (Elementa, p. 253.)
* What has been called the cupula of the Yew is said by Schleiden to be a late developement of the primine of the ovule.