Графични страници
PDF файл
ePub

however is certain with respect to this fluid, that without detri. ment to the plant, it yields to the industrious bee the material for the manufacture of honey, a luxury highly valued from the most ancient times. Virgil knew that bees made honey from the juices which they gathered from flowers; and we indeed, on this subject, know but little more than he has beautifully ex. pressed in his pastorals.

Although we are always discovering something new and wonderful in the economy of nature, and in some cases seem permitted to search into the hidden mysteries of her great Author, yet in our researches we are ever made sensible of the limited nature of our own faculties; and a still, small voice, seems to whisper to man in the proudest triumphs of his reason, “Hitherto shalt thou go, but no farther.”

LECTURE XIII.

Stamens and Pistils.

ALTHOUGH the calyx and the corolla may be wanting, the stamens and pistils are indispensable to the perfection of the fruit. They are in most plants enclosed by the same envelope, or stand upon the same receptacle; in the class Monæcia they are on different flowers which spring from one common root; and in Diæcia, they are on different flowers, springing from different roots. Yet however distant the stamens and pistils may be, nature has provided ways by which the pollen from the staminate flowers may be conveyed to the pistillate, and there assist in perfecting the seed. That you may the better understand this curious process, and the organs by means of which it is carried on, we will examine each one separately.

Stamens. Stamens are thread-like parts which are exterior with respect to the pistil, interior with regard to the corolla. They exhibit a variety of positions with respect to the pistil. These positions seldom

vary in the same family, and they have therefore been taken by the celebrated Jussieu as one of the fundamental distinctions in his classification, called the natural method. If the stamens are inserted upon the pistil, as in umbelliferous plants, they are said to be epigynous (from epi upon, and gynia - pistil); if the stamens are inserted under the germ, as in cruciform plants, they are said to be hypogynous (from hypo under, and gynia pistil); when the stamens are inserted upon the calyx,

Reflections Stamens and pistils necessary---Definitions of the stamen-Positions with respeet to the pistil.

and thus stand around the germ, as in rosaceous plants, they are said to be perigynous, (from peri around, and gynia pistil).

When a corolla is monopetalous, the number of stamens ist usually either equal or double, or half that of the divisions of the corolla ; the stamens in such flowers never exceed twenty.

In polypetalous corollas, the number of stamens may be much greater. When the number of stamens equals the divisions of the corolla, they usually alternate with these divisions. When the number of stamens is double the divisions of the corolla, half of the stamens are usually placed in the intervals of the divisions, and the remaining half before each lobe of the corolla, corresponding to the intervals in the divisions of the calyx. If any of the stamens are barren or without anthers, they are those which are placed before the lobes of the corolla.

In commencing the analysis of flowers according to the Lin. næan system, you learned that the number of stamens, their position, relative length, and connection, taken either singly or in combination, afford certain and distinctive marks for purposes. of classification.

In the first place we find the stamens differing in number, in different plants; some plants have but one, some two, and so on till we come to ten; when they have more than ten, we find the number in the same plant varies, and therefore we cannot depend on the cirumstance of number for further classification.

We then resort to position, and consider whether the stamens are inserted upon the calyx or not, thus furnishing an eleventh and a twelfth class.

Inequality in the length of stamens, considered with respect to number, furnishes us with a thirteenth and fourteenth class.

The connection or union of stamens gives us the fifteenth class, where the filaments of the stamens are united in one set; the sixteenth class where they are in two sets; the seventeenth where the anthers of the stamens are united.

The three remaining classes of phenogamous plants are distinguished by the position of the stamens with respect to the pistils. In the eighteenth class the stamens stand on the pistil; in the nineteenth the stamens and pistils are on separate flowers on the same plant; in the twentieth they are on separate plants; and in the twenty-first they are invisible.

We will now proceed to the parts of the stamen; these are two. The filament and anther.

The Filament, is so called from filum, a thread. Filaments

Divisions of monopetalous corollas usually in proportion to the number of stamens - Situation of the stamens with respect to the divisions of the corollar -Stamens used for purposes of classification-Differ in number-In the mode of insertion-Stamens differ in length-In connection-In position with respect to the pistil-Parts of the stamens—Filament.

vary in their form; some are long and slender, as in the pink ; others are short and thick, as in the tulip. They are usually smooth, but in the mullein they are bearded; in the spider-wort, (Tradescantia,) they are covered with down. In most cases a filament supports but one anther, but sometimes it is forked and bears two or more ; in some instances, many filaments have but one anther. When the filaments are enclosed in the tube of the corolla they are said to be inserted, when they extend out of it, exserted. In some cases the filament is wanting, and the anther is sessile, or immediately attached to the corolla.

In double flowers, the stamens, which seem to be intimately connected with the parts of the corolla, are changed to petals.

This is the effect of cultivation, which by affording the stamens excess of nourishment, causes them to swell out, and thus as. sume the form of petals. In some double flowers almost every trace of the stamens disappears; in others, it is very easy to perceive the metamorphosis which they have undergone, as they retain something of their original form. The anthers usually disappear, which shows that the filaments have absorbed all the nourishment. In many double flowers, roses especially, we can see the change as it takes place, some stamens entirely changed, others retaining something of their form, and others still perfect. When all the stamens disappear, no perfect fruit is produced. On account of this degeneration of the stamens, cultivated flowers are not usually so good for botanical analysis, as wild ones. The single flower exhibits the number of parts which nature has given to it. The rose in its native state has but five petals.

Anther, is a little knob or box usually situated on the summit of the filament; it has cells or cavities which contain a pow. der called the pollen ; this is yellow, and very conspicuous in the lily and tulip. You have here the representation (Fig. 60) Fig. 60.

of a stamen with its filament (a), its anther (6), and the discharging pollen (c). In many flowers you will perceive the filament to be wanting; the anthers are then said to be sessile; that is, placed immediately

upon the corolla ; as at (d), which d represents a flower cut open, and its

five stamens growing sessile in the throat.

Pistil. In the centre of the flower stands the pistil, an organ essential to the plant. Like the stamens, the pistils vary in number

Stamens changed to petals—Anther-Pistil, situation and number.

[graphic]

to vary

IIS

the germ.

in different plants, some having but one and others hundreds. Linnæus has founded the orders of his first twelve classes on the number of these organs. When they are more than ten, he does not rely upon their number, which in this case is found

in individuals of the same genus. The pistil consists of three parts, the germ, style and stigma. It may be compared to a pillar; the germ (Fig. 61, a), corresFig. 61. ponding to the base ; the style (b) to

the shaft; and the stigma (e) to the capital.

The figure at (g) represents the pig. til of the poppy; the germ or base is very large; you will perceive that the style is wanting, and the stigma is sessile, or placed immediately on

The style is not an essential part, but the stigma and germ are never wanting ; so that these two parts, as in the poppy, often constitute a pistil.

Germ. The germ contains the rudiments of the fruit yet in an embryo or unformed state. A distinction is to be made between the germ here spoken of and the germ of the bud. This germ is the future fruit, and in passing to the state of the mature fruit, it undergoes a very great change. You would scarcely believe that the pumpkin is but the germ of the small yellow flower of the plant. The germ is said to be superior when placed above the calyx or corolla, as in the strawberry: inferior when below them, as in the apple. The figure of the germ is roundish in some plants, cordate and angled in others; but its various forms can better be learned by observation than description.

Style. This, like the filament, is sometimes wanting ; when present, it proceeds from the germ, and bears the stigma on its summit. It is usually long and slender, of a cylindrical form, consisting of bundles of fibres, which transmit from the stigma to the germ the fertilizing pollen.

Stigma. This word signifies a perfecting. The stigma is the top of the pistil, and always present; if the style be wanting, it is placed upon the germ, and said to be sessile, as in the tulip. The stigma is various in size and form; sometimes it is a round head; sometimes hollow and gaping, more especial. ly when the flower is in its highest perfection; it is generally downy, and always more or less moist with a peculiar viscid Auid.

[graphic]

Orders founded upon the pistil--Parts of the pistil— Pistil compared to a pillar-Germ-Style-Stigma.

Use of the Stamens and Pistils.

In a former part of our lectures, it was observed that the stamens and pistils were necessary to the perfection of the fruit; we will now explain to you the manner in which they conduce to this important object; as you are now acquainted with the different organs and their names, you will no doubt easily understand the explanation.

The pollen, which in most flowers is a kind of yellow dust, is thrown out by the bursting of the anther, which takes place in a certain stage of the flower. The pollen is very curiously form. ed; although appearing like little particles of dust, upon examining it with a microscope it is found to be composed of innumerable organized corpuscles. These little bodies, though usually yellow, are sometimes white, red, blue, &c. In order to observe them well, it is necessary to put them upon water; the moisture, by swelling them, renders their true form perceptible. They are oblong in the Umbelliferous plants, globu. lar in the Syngenesious, and triangular in some others. In some their surface is smooth, in others armed with little points. They are connected together by minute threads, as in the honey-suckle, &c. These little bodies, thus placed upon water, swell with the moisture until they burst; a liquid matter is then thrown out, and, expanding upon the surface of the water, appears like a light cloud.

“I should never finish,” (says the French botanist,* from whom this account of the pollen is translated,) "if I should attempt to describe the varieties of appearances in the pojlen.” If

you have paid attention to what has been said respecting the pollen, you perceive that wonders exist in nature, which are entirely unperceived by a careless observer. You would scarcely have imagined that the yellow dust seen upon the lily or tulip, and scarcely visible upon many other flowers, should exhibit appearances so interesting, as to engage the attention of a learned philosopher to such an extent, and in such a num. ber of experiments, that he should find it too long a task to enumerate all the phenomena which he had observed. It is to convince you that the field of observation in the works of nature is absolutely unbounded, that we have brought this subject before you ; for in general our limits do not permit us to penetrate into the minute investigations which delight those who have passed beyond the first principles of natural science. Another purpose,

and one more connected with our present * Mirbel.

Use of the stamens and pistils Description of the pollen--Why the subject is introduced.

« ПредишнаНапред »