« 上一頁繼續 »
spreading out from a common centre, like the rays of an umbrella, bearing flowers on their summits; as fennel and carrot.
6th. Cyme (Fig. 64, c), resembles an umbel in having its common stalks all spring from one centre, but differs in having those stalks irregularly subdivided; as the snow ball and elder.
7th. Corymb (Fig. 65, a), or false umbel, when the peduncles rise from different heights above the main stem, but the lower ones being longer, they form nearly a level or a convex top;, as the yarrow.
8th. Fascicle (Fig. 65, 6), flowers on little stalks variously inserted and subdivided, collected into a close bundle, level at the top; as the sweet william; it resembles a corymb, but the flowers are more densely clustered.
9th. Head (Fig. 65, c), or tuft, has sessile flowers heaped together in a globular form; as in the clover, and button bush (cephalanthus).
10th. Ament or catkin, is an assemblage of flowers, composed of scales and stamens, arranged along a common thread-like receptable, as in the chesnut and willow; this is more particularly described under the divisions of the calyx. The scales of the ament are properly the calyxes; the whole aggregate, including scales, stamens or pistils, and filiform receptacle, con
stitutes the ament. At Fig. 66 is the rep-
taining pistillate flowers; this is oblong,
calyx is a flat scale, with deep, fringed
fertile or pistillate flower; the calyx or bract is a little below the corolla, which is cup shaped, of one petal, and crowned with an egg-shaped, pointed germ; the germ is superior, and bears four (sometimes eight) stigmas.
The staminate ament resembles the pistillate, except that its corolla encloses eight stamens, but no pistil. The poplar is in the class Diæcia, because the pistillate and staminate flowers are on different trees, and of the order Octandria, because its barren flowers have eight stamens.
11th. Spadix, is an assemblage of flowers growing upon a common receptacle, and surrounded by a spatha or sheath, as in the Egyptian lily. At Fig. 67 a, is a representation of the blossom of the wild
turnip (arum); a repb
resents the spatha which is erect, sheath
ing, oblong, convolute a b
at the base, b; com.
pressed above and beB
low the middle; c represents the spadix
which, from its clubshaped appearance, is called clavi form (from clava, a club).
At B is the spadix divested of the spatha; a is the clavi. form summit; ba ring of filaments without anthers ; c a ring of sessile anthers; d a dense ring of pistillate flowers with sessile stigmas; each germ produces a one-celled globular berry. This plant is of the class Monæcia because its staminate and pistillate flowers are separate, but yet grow on the same plant; it is in the order Polyandria, because its stamens are numerous.
Receptacle. The receptacle is the extremity of the peduncle; at first it supports the flower, and afterwards the fruit. As this is its on. ly use, it may properly be considered in connexion with the organs of fructification. In simple flowers, as the tuļip, the receptacle is scarcely to be distinguished from the peduncle, but in compound flowers it is expanded and furnishes a support for the flowers and fruit. Receptacles are of various kinds; as,
1st. Proper, supports but one flower, as in the violet and lily.
2d. Common, supports many flowers or florets, the assemblage of which forms an aggregate or compound flower, as in the sunflower and dandelion. The common receptacle presents a great variety of forms; it is either dry or pulpy: it is concave in the artichoke; convex, in other plants ; flat, in the sunflower, conical in some, and spherical in others. As to its surface, it is punctaie, or interspersed with hollow points or dots, as in the daisy, hairy as in the thistle, naked as in the dandelion, or chaffy as in the chamomile.
3d. Rachis, is the filiform receptacle which connects the fo. rets in a spike, as in the heads of wheat.
The Fruit. The fruit is composed of two principal parts, the pericarp and seed. The term pericarp is derived from peri around, and karpos seed or fruit; it signifies surrounding the seed. All that in
fruit which is not the seed belongs to the pericarp. Let us now inquire into the progress of the fruit from its first appearance in the germ to its mature state.
When you analyze a flower, you often find it necessary to ascertain the num. ber of cells contained in the germ. In making this examination what appearance did the interior of the germ present, when exposed by cutting it horizontally? You saw there minute bodies of a pale green colour, and an apparently homogeneous
Receptacle—Different kinds of receptacles--Fruit, the two principal partsDerivation and signification of the word pericarp-Progress of the fruit considered.
nature; each of these is called an ovule,* and their outer cover ing, an ovary. These ovules, before the fertilization of the germ by the pollen, are scarcely perceptible; after this period, and the fading of the corolla, the ovules increase in size, and the embryo and other parts which constitute the seed become manifest. The ovary enlarges with the growth of the ovules; the use of this covering is not confined to the mere protection of the seeds from injury, but it is furnished with glands, which secrete such juices as are necessary for the growth and developement of the ovules. As the ovary becomes more mature it takes the name of pericarp. Pericarps in their growth be. come either woody or pulpy; the latter absorb oxygen gas and throw off carbonic acid; saccharine juices are elaborated in their cellular integument. In another stage, the pulpy substance passes through a slight fermentation, the organization is affected, the juices sour, the pulp decomposes and putrefaction ensues. Such is the change which you may see in the orange, apple, &c. during their progress towards maturity and decay.
Pericarp and Seed. The germ being fertilized, the parts of the flower which are not necessary for the growth of the fruit, usually fade, and either fall off, or wither away. The germ continues to enlarge until it arrives at perfection. Every kind of fruitt you can see has been once but the germ of a flower. The size of fruit is not usually proportioned to that of the vegetable which produ. ced it. The pumpkin and the gourd grow upon slender her. baceous plants, while the large oak produces but an acorn.
Every pericarp is formed of a parenchymous substance; this substance is surrounded externally by a cuticle called the epicarp, internally by a membrane called the endocarp, and an intermediate part called the mesocarp. In a peach, for example, the kernel is the seed; the fleshy substance the pericarp; the skin is the epicarp; the pulpy cellular substance, which absorbs a great quantity of sap and constitutes the principal part of the fruit, is the mesocarp; the shell, deprived of moisture and nourishment by the absorbtion of the mesocarp, and thus contracted in its fibres and rendered dry and tough, is the en. docarp.
* From ovum, an egg.
+ The term fruit, in common language, is limited to pulpy fruits which are proper for food; but in a botanical sense, the fruit includes the seeds and pericarps of all vegetables.
Ovules—Ovary--Use of the ovary—Its name in a mature state-Pulpy pericarps—Germ-Size of fruit not in proportion to the plant that produces itEpicarp-Endocarp—Mesocarp.
The pericarp consists of different parts, as,
1st. Valves or external pieces, which form the sides of the seed vessels. If a pericarp is formed of but one piece it is uni. valved; the chesnut is of this kind. A pericarp with two valves is said to be bivalved, as a pea-pod. The pericarp of the violet is trivalved, that of the stramonium quadrivalved. Most valves separate easily when the fruit is ripe ; this separa. tion is known by the term dehiscence.
2d. Sutures or seams, are lines which show the union of valves; at their seams the valves separate in the mature stage of the plant; they are very distinct in the pod which has two.
3d. Partitions or dissepiments, are internal membranes which divide the pericarp into different cells: these are longitudinal when they extend from the base to the summit of the pericarp; they are transverse when they extend from one side to the other.
4th. Column or columella, the axis of the fruit; this is the central point of union of the partitions of the seed vessels; it may be seen distinctly in the core of an apple. This was noticed under the head of receptacles; it is the receptacle of the fruit.
5th. Cells, are divisions made by the dissepiments, and con. tain the seeds; their number is seldom variable in the same genus of plants, and therefore serves as an important generic distinction.
6th. Receptacle, is that part of the pericarp to which the seed remains attached until its perfect maturity; this organ, by means of connecting fibres, conveys to the seed for its nour. ishment juices elaborated by the pericarp.
Some plants are destitute of a pericarp, as in the labiate flowers, the compound flowers, and the grasses ;
in these cases the seeds lie in the bottom of the calyx, which performs the office of a pericarp.
Mirbel's classification of Fruits or Pericarps. Mirbel has divided the fruits of all phenogamous plants into two classes; lst. gymnocarps, which include all such as are not masked or covered by any strange organ, or form no union which conceals their true character. 2nd. angiocarps, which include all fruits covered by any strange organ, which disguises them from observation.
Valves-Sutures—Partitions, or dissepiments--Column-Cells-Receptacle of the pericarp-Pericarp sometimes wanting—Mirbel's two grand divisions of fruit.