THE PARACHUTE OF THE

DANDELION.

HE velocity of a falling body is determined in accordance with

THE of a

a well-known law. During the first second it passes through a space of sixteen feet, and acquires a velocity of thirty-two feet. Its speed goes on increasing uniformly, an additional velocity of thirtytwo feet being imparted each second the body continues to fall. This law holds good for all bodies, large or small, light or heavy, but applies strictly, only when the descent occurs in a vacuum. Under the exhausted receiver of an air-pump a guinea and a feather liberated simultaneously reach the bottom of the receiver at the same instant. If the atmosphere were removed, a pound of lead and a pound of feathers would fall to the earth in the same time. In the atmosphere, dense, solid bodies approximate closely to the velocity they would acquire in vacuo. If, however, the figure of the falling body be such, that it presents an expanded horizontal surface large relatively to its weight, then its downward progress is much impeded by the resistance of the air. The density of the atmosphere increases as we approach the earth's surface; a falling body must therefore meet increasing resistance, and if the column of air which it displaces be only thick enough, instead of obeying the law of acceleration, it will descend with a constantly diminishing speed. Taking advantage of this circumstance, aeronauts descend safely from altitudes of several thousand feet. The parachute, by means of which these descents are effected, is simply an enormous umbrella, so constructed that the air expands it in its descent. Its large surface meets with so much resistance in passing down through the air, that the aeronaut is enabled to descend with safety, and alights gently on the earth. Recently, the parachute has been brought prominently into notice, and, notwithstanding one or two unfortunate accidents, the practicability of this appliance has been thoroughly demonstrated.

The force of gravity tends to impart to a falling body a velocity of thirty-two feet every second during which it operates upon that

The Parachute of the Dandelion.

23

body, and a parachute is merely a contrivance for diminishing this velocity.

In the seeds and fruits of many plants we find interesting applications of this principle. Plants differ from animals in this respect, that while the latter are free to move about from place to place, plants, as a rule, are fixed to one spot. The egg of a bird or reptile is in many ways analogous to a vegetable seed; but while an animal, in virtue of its locomotive power, can deposit its eggs where it pleases, a plant is unable to do so with its seeds. Moreover, young birds or reptiles after they are hatched, having power to move about, can disperse themselves in search of food and other requirements. When a seed, on the other hand, germinates, the young seedling, unless in one or two very exceptional cases, has no power to change its place. For this reason seeds are furnished with appliances for securing their dispersion unnecessary in the case of an egg. Were a plant to let its ripe seeds fall straight to the earth, the resulting seedlings would be so crowded that hardly any of them could attain maturity.

The natural agency of which plants most frequently avail themselves for the dissemination of their seeds is the wind. Now, the distance to which a seed will be carried depends on two things: first, the extent of surface exposed to the lateral force of the wind; secondly, the length of time during which the wind can act on that surface. The second of these, all things considered, is the more important factor. A slowly-falling seed has a better chance of being blown away than one which falls more rapidly, for the latter, even if it should present a larger surface to the wind, runs more risk of falling while there is no wind. The longer a seed takes to fall the less likelihood is there that the air will continue motionless until it reaches the earth. Although, then, we do very often meet with fruits and seeds which expose an expanded lateral surface to the wind, contrivances which act by retarding the rapidity of their fall are equally common. Both provisions frequently occur together. Thus the fruits of the ash, maple, and plane have expanded membranous wings, and in well-developed specimens the wings are seen to be obliquely twisted. The fruit of the sycamore strongly resembles the screw-propeller of a steamship, and in descending it acquires a rotatory motion. The membranous wing attached to the fruit of the lime acts in a similiar manner. The object of this oblique twisting of the wing is to diminish the velocity of the descending fruit. Again, there are seeds so shaped that when dropped from the hand they rarely fall straight to the earth, but shoot aside in a slanting direction. This may be seen when a handful of the crescent-shaped seeds of the

arrow-grass are slowly let fall. Winged seeds flutter in their descent, and, like the falling leaves described by Wordsworth,

Eddying round and round, they sink,
Softly, slowly, one might think,
From the motions that are made,
Every little leaf conveyed
Sylph or fairy hither tending,
To his lower world descending,
Each invisible and mute

In this wavering parachute.

In a variety of ways the velocity of a falling seed may be lessened, and of these we have a curious and interesting example in the parachute of hairs attached to the fruit of the dandelion.

Like the daisy, sunflower, and thistle, the dandelion belongs to the great order compositæ. The members of this order are distinguished by their peculiar inflorescence. What most people call the flower in the daisy and dandelion is not a single flower, but an inflorescence or collection of florets, seated on the flattened summit of the stalk, and surrounded by a circle of green scales or bracts. This contracted inflorescence is called a capitulum, and the bracts surrounding it constitute the involucre. On account of this crowding together of small flowers, whereby the inflorescence is made to resemble a single large flower, the sepals of the individual florets are not required. These, in ordinary flowers, form the calyx or outer circle of green, leaf-like organs which protect the other parts. In composites the sepals are not necessary, for the involucre of bracts discharges their office and protects all the florets on the capitulum. Instead of green sepals, then, we find outside the corolla of each floret in the dandelion a circle of hairs. As the fruit ripens these hairs. become very much developed and constitute the pappus-a structure very characteristic of the composite order, though absent in the daisy, nipplewort, and some others.

The sessile pappus is the more usual form, but in the dandelion each shuttle-shaped fruit terminates above in a slender beak, which forms the handle, so to speak, of the inverted brush. When the pappus hairs are stalked in this way, the capitulum produces a feathery sphere or "clock." If the pappus be sessile each fruit presents the appearance of a shuttlecock, and the ripe capitulum resembles a mop. In the botanical name for groundsel-Senecio, from senex-there is an allusion to the hoary pappus.

The bodies distributed by the aid of these hairs in the order compositæ are achenes, as the dry one-seeded fruits are called. The

resemblance between a fruit of this description and a seed is so close, however, that for our present purpose the distinction may be neglected.

That the pappus plays an important part in the life-history of the dandelion, might be inferred from the precautions taken by nature to secure its perfect development. While the fruit is maturing, the bracts close up and cover in the florets, just as they did before the flowers expanded; the capitulum, in fact, re-assumes the appearance it had in the bud. Opening the quiver-like involucre at this stage, we find it full of fruits, crowded together on the receptacle. Each achene is tipped with a pencil of silky hairs, which becomes elevated as the apical beak of the fruit elongates. When the fruits are fully developed, the protecting bracts fold back, the receptacle, till now concave, becomes convex-the involucre cup is turned inside out in fact, causing the fruits to assume different inclinations, so that their beaks stand at an equal distance from one another. At the same time the vertical pappus hairs spread out till they almost stand at right angles to the beak. The brushes are thus converted into parachutes, and so arranged that they form a sphere. All these changes occur in co-ordination, and are executed with the utmost nicety and precision in an incredibly brief space of time. Thus there is evolved the beautiful feathery and symmetrical globe of fruit so familiar to everyone as the dandelion "clock." While these changes are in progress, the flowerstalk becomes erect, the better to expose the seeds to the action of the wind. Under its influence the hairs of the pappus are still further dried, and the connection of each fruit below with the receptacle is gradually weakened, until at last it gives way and the seeds are scattered on the breeze.

Without this provision the achenes of the dandelion would fall straight and quickly to the earth, forming a little heap at the base of the stem. So well, however, do these hairs serve their purpose that even in still air an achene falls very slowly, and the slightest current is sufficient to bear it a long distance away. The hairs on the top of the beak are not quite horizontal, but slope slightly in an upward direction; they thus present a lateral surface which causes the fruit to be borne faster and farther before the wind. A stalked pappus is better exposed than a sessile tuft of hairs; the slender beak therefore serves, like the straightening of the flower-stalk, to give the fruits a fair start. The arrangement in goat's-beard is very similar, but the achenes are larger, their beaks longer, and the feathery globe or "clock" is four or five times bigger than that of the dandelion.

It is a distinct disadvantage if fruits get detached before they are

perfectly ripe. But this is hardly possible, since the weakening of the attachment is only brought about by the wind desiccating their tissues. Wind is also necessary to dry the pappus. The wind must therefore mature the fruits before it can detach them. There is very little chance of their ripening or falling off unless when there is some wind. The fruit thus attains maturity at a time most favourable to dispersion.

The parachute, by which the fall of the seed is retarded, is not, however, the only point in which the arrangements for the dispersion of the dandelion resemble those adopted by the balloonist. The seeds are so light that there is a possibility of their being carried too far. After carrying them a sufficient distance from the mother plant the wind, instead of dropping them to the earth, might bear them aloft again, and the seeds be thus driven about unnecessarily— Like long-tail'd birds of Paradise

That float through Heaven, and cannot light.

An aeronaut as he nears the earth must let go his grappling iron or his balloon is in danger of being blown about and perhaps carried out to sea. For similar reasons it is of advantage to a wind-driven seed if, when nearing the earth, it has some provision by which it can anchor itself and come to rest. This, without doubt, is the meaning of the little projections or prickles with which the lower solid portion of the dandelion achene is crowned. When the seed alights from its aerial voyage these minute prickles very readily catch hold of any grass stem or similar object with which they happen to come in contact, and the seed is retained, the wind being unable to lift it again. We have seen the dandelion seed moor itself in this way to a piece of worsted lying on a garden walk.

After a seed has alighted, it may experience a further difficulty in reaching the soil if this happens to be coated with matted vegetation. In such circumstances the plumes are apt to prove an encumbrance. For this reason the seed of the thistle after a time, probably as the result of further desiccation, detaches itself from its pappus and falls to the earth. The hairs, having served their purpose, are discarded. But in the dandelion the hairs remain attached to the seed after it has alighted. A shower of rain might destroy the pappus hairs, and so assist the seed in penetrating to the soil. But we have just seen that the achenes are not likely to be scattered in wet weather, and so a further provision becomes necessary. The slender beak, which serves in the first instance to expose the pappus to the wind, now comes into requisition again and assists the seed in making its way