Stigma (botany)

“ When the stigma is ready, an unopened anther from the desired flower is crushed on the finger nail or a knife blade, and the pollen is rubbed on the stigma by means of a tiny brush, the point of a knife blade, or a sliver of wood. The flower is again covered with the bag, which is allowed to remain for several days until all danger of other pollination is past. Care must be taken completely to cover the stigmatic surface with pollen, if possible. The seeds produced by a crossed flower produce hybrids, or plants having parents belonging to different varieties or species. ”

“ Fig. 195.—Diagram to represent Fertilization.
s, stigma; st, style; ov, ovary; o, ovule; p, pollen-grain; pt, pollen-tube; e, egg-cell; m, micropyle.
In order that the pollen may grow, the stigma must be ripe. At this stage the stigma is usually moist and sometimes sticky. A ripe stigma is said to be receptive. The stigma may remain receptive for several hours or even days, depending on the kind of plant, the weather, and how soon pollen is received. Watch a certain flower every day to see the anther locules open and the stigma ripen. When fertilization takes place, the stigma dies. ”

“ The pollen and the stigma are always in perfection at the same time, the latter commonly withering and falling off a little after the anthers, though the style may remain to become an useful appendage to the fruit. The Viola tricolor or Pansy, the Gratiola, the Martynia, and many plants besides, have been observed to be furnished with a stigma gaping only at the time the pollen is ripe. ”

“ The globe has an opening in front, under which is a kind of lip, which looks like a shutter let down to show the opening; and though, from its thick fleshy nature, it looks like a stigma, it is only the outer covering of that organ, for the stigma lies within the opening. In this manner the stigma and anthers are completely concealed; and thus it will be seen, that nothing can be more complex and intricate than the construction of the flowers of the Heartsease. ”

“ But of all flowers that of the Barberry-bush, t., is most worthy the attention of a curious physiologist. In this the six stamens, spreading moderately, are sheltered under the concave tips of the petals, till some extraneous body, as the feet or trunk of an insect in search of honey, touches the inner part of each filament near the bottom. The irritability of that part is such, that the filament immediately contracts there, and consequently strikes its anther, full of pollen, against the stigma. ”

“ Other very curious changes occur in flowers of different species during the reproductive act. The stigma is observed to become moistened, and even to become distinctly odorous. Often, too, it becomes intensely congested with the juices of the plant, and sometimes even acquires an uncommon and most remarkable degree of contractility. This is the case with the stigma of the tulip and one variety of sensitive plant, and is in these plants observed to occur not only after the application of the pollen to the stigma, but when excited by any other means of stimulation. ”

“ From the structure of the flowers it would seem that self-pollination would be impossible. When the flower is open, the stigma has never been observed to be in contact with the terminal portion of the large stamen. The stamens do not dehisce until after the flower has opened, nor does the stigma come in contact with the tip of the anther in the bud ”

“ The lobes of a stigma are flat and pointed as in Mimulus and Bignonia, fleshy and blunt, smooth or granular, or they are feathery, as in many grasses (fig. 105) and other wind-pollinated flowers. In Orchidaceae the stigma is situated on the anterior surface of the column beneath the anther. In Asclepiadaceae the stigmas are united to the face of the anthers, and along with them form a solid mass.
The ovule is attached to the placenta, and destined to become the seed. ”

“ The structure and development of the flower are such that while occasionally, particularly in healthy plants out of doors, the stigma becomes receptive and takes the pollen as it is pushed out through the stamen tube by the elongating style, it is more often pushed beyond them before the pollen matures, so that the pollen has to reach the stigma through some other means. ”

“ The adaptations found in the female organs are chiefly such as increase the surface capable of receiving the pollen, and therefore increase the chance of fertilisation. A big stigmatic surface is common: not only is the receptive part of the style large, but it usually bears very large stigmatic papillæ, which gives a velvety hoary look to this type of stigma. In the grasses the three divisions of the stigma are always more or less conspicuous ”

“ No doubt you will be wondering why a flower is not fertilised when anthers covered with pollen surround the stigma. The explanation is very simple, for the stigma has to become ripe before it can receive any pollen. In some plants the stigma is ripe before the anthers give off pollen, whilst in others all the pollen is given from off the anthers before the stigma becomes ripe. Thus we see how Nature prevents a flower from fertilising itself. ”

“ An œconomy, in many respects similar to that now described, obtains also in Orchideæ, in which, however, the processes connecting the antheræ with the stigma, where they exist, are in many cases derived from the masses of pollen themselves; but in others they as certainly originate from the stigma, or its glandular appendage. ”

“ The stamens surround the style, the anthers so closely touching each other laterally that they enclose a space in which the ovary and style occupy the centre, and from which 5 the stigma protrudes. The anthers shed their pollen, which is dry, into this space. Two of the stamens send out each a long tail into the hollow petal-spur, which secretes honey from its tip. The reason why the flower-stalk bends over is, that the stigma may hang down instead of being erect. A bee smells the honey and alights on the odd petal. ”

“ As the sepals fall early, the white stamens and stigmas are the most conspicuous parts of the flowers. A cluster opening its blossoms almost simultaneously, the plant's only hope of cross-fertilization lies in the expectation that the small female bees (Halictus) which come for pollen - no nectar being secreted - will leave some brought from another flower on the stigma as they enter, and before collecting a fresh supply. The time elapsing between the maturity of the stigmas and the anthers is barely perceptible ”

“ Darwin showed that the external differences are correlated with others in the structure of the stigma and in the nature of the pollen. The long-styled flowers have a spherical stigma provided with large stigmatic papillae; the pollen grains are oblong and smaller than those of the short-styled flowers. The number of the seeds produced is smaller and the ovules larger, probably also fewer in number. The short-styled flowers have a smooth compressed stigma and a corolla of somewhat different form; they produce a greater number of seeds. ”

“ None of it, however, can get upon the stigmas, for they are not yet exposed. After a short time has elapsed, during which much of the pollen has probably been rubbed off, the style is seen to split at the top; and as the halves separate and roll back, Fig. 3, their inner faces (the stigmas) are exposed. If, now, the flower be visited by an insect which has previously been to a younger flower, the pollen he brings will be deposited upon the stigmas as he rubs against them, and cross fertilization will be effected. ”

“ As these flowers set no seed without insects' aid, the provisions made to secure the greatest benefit from their visits are marvelous. Of the three kinds of blossoms, one raises its stigma on a long style reaching to the top of the flower; a second form lifts its stigma only halfway up, and the third keeps its stigma in the bottom of the tube. Now, there are two sets of stamens, three in each set bearing pollen grains of different size and value. ”

“ The stigma is not always ready to accept the pollen, but when it is ready it becomes moist, and in some flowers sticky, which shows it is in condition to accept the pollen or is ready to mate and that the seeds are ready for their development. ”

“ Their habit of closing both in the evening and in rainy weather indicates that they are adapted for diurnal insects only; but if the bell hang down, or if the corolla drop off, the pollen must fall on the stigma and effect self-fertilization. Many more insects visit this flower than the large bindweed, attracted by the peculiar fragrance, and led by the white streaks to the orange-colored under surface of the ovary, where the nectar lies concealed. Stigmas and anthers mature at the same time ”

“ But in a short time five stamens begin to raise themselves and cling round the stigma or knob at the top of the seed-vessel, as in the middle flower. Now you would think they would leave their dust there. But no! the stigma is closed up so tight that the dust cannot get on to the sticky part. Now, however, the bee can get at the honey-glands on the outside of the raised stamens; and as he sucks it, his back touches the anthers or dust-bags, and he carries off the pollen. ”

“ The anther, which opens towards the outside, is sheltered beneath the over-arching style arm below the stigma, so that the insect comes in contact with its pollen-covered surface only after passing the stigma, while in backing out of the flower it will come in contact only with the non-receptive lower face of the stigma. ”

“ The stigma of an ordinary grass consists of two divaricating plume-like structures composed of thin-walled cells. When the paleæ open these stigmatic plumes protrude, one on either side, and readily catch pollen shed from the dangling stamens and carried by the wind, and since the pollen of the same flower is usually shed at a time when the stigmas of many neighbouring plants are mature, there is every opportunity for cross-fertilisation. ”

“ It should be noticed that an insect in making its escape, must first brush past the stigma and afterwards one of the anthers, so that it cannot leave pollen on the stigma, until being already smeared with pollen from one flower it enters another ”

“ You will notice that all this time the be does not touch the sticky stigma which hangs high above her, but after the anthers are empty and shrivelled the stalk of the stigma grows longer, and it falls lower down. By-and-by another bee, having pollen on her back, comes to look for honey, and as she goes into No. ”

“ I then searched for the stigma, and found it outside the indusium with the pollen-tubes penetrating it; and I convinced Dr. Hooker that botanists were quite wrong in supposing that the stigma lay inside the indusium. In Scaevola microcarpa the structure is very different, for the immature stigma lies at the base within the indusium, and as the stigma grows it pushes the pollen out of the indusium, and it then clings to the hairs which fringe the tips of the indusium; and when an insect enters the flower, the pollen (as I have seen) is swept from these long hairs on to the insect's back. ”

“ When the little craft happen to get stranded in the recesses of a female Valisneria flower, they adhere to the tri-lobed stigma, and some of the pollen-cells are sure to be left sticking to the fringes on the margins of the stigmatic surface.” [9]
This is a good example of purely organic behaviour admirably adapted to secure a definite and important biological end. Few will be likely to contend that it is even accompanied by, still less under the guidance of, any conscious foresight on the part of the plant. ”

“ But, in the first place, it appears that there is a natural aversion in plants, as well as in animals, to irregular sexual unions; and in most of the successful experiments in the animal and vegetable world, some violence has been used in order to procure impregnation. The stigma imbibes, slowly and reluctantly, the granules of the pollen of another species, even when it is abundantly covered with it ”

“ Our curiosity is rewarded by the discovery that the lobes of the stigma are so tightly pressed together that they can at first receive no pollen upon their sensitive surfaces. We also find that the anthers open only by a pore at their tips, and when irritated by the jar of a visiting bee, discharge their pollen upon its body through these outlets. This being accomplished the fringed stigma pushes forward, brushing aside whatever pollen may have fallen within the tube. When it finally projects beyond the anthers, it opens, and is ready to receive its pollen from the next insect-visitor. ”

“ After the anthers have shed their pollen and become effete, the stigma matures, and occupies their place. By this time the flower presents a wider entrance, and as the moisture-loving plant keeps the nectaries abundantly filled, what is to prevent insects too small to come in contact with anthers and stigma in the roof from pilfering to their heart's content? The woolly throat discourages many, to be sure ”

“ Several flowers on the showy spike attract numerous insects. One alighting on the lower lip must thrust his tongue beneath the upper one to reach the nectar in the spur, passing on its way the irritable stigma, which receives any pollen he has brought in. Instantly it is touched, the stigma folds up to be out of the way of the tongue when it is withdrawn from the spur now laden with fresh pollen. It is thus that self-fertilization is escaped. Many vigorous seeds follow in each capsule. ”

“ May 28th, 1861, page 151:—"Leschenaultia formosa has apparently the most effective contrivance to prevent the stigma of one flower ever receiving a grain of pollen from another flower; for the pollen is shed in the early bud, and is there shut up round the stigma within a cup or indusium. ”