To pass on. Some seeds, many of them of considerable size as compared with those which we have just considered, have coverings which are furnished with a membranous wing ([Fig. 13], d), sometimes extending all round the seed, as in the Elm (Ulmus), more often placed at one side, as in the Sycamore (Acer). The effect of such wings is to reduce the rate of fall, imparting to the seed an irregular zigzag motion, as in the former case, or a spinning motion as in the latter. A Sycamore seed with the wing removed will fall four or five times as fast as with the wing present. But while a well-developed wing forms a more efficient dispersal device than mere reduction in size as found in Seed Plants, the rate of fall of wing seeds as a whole shows that these appendages do not fit them for anything but short voyages.
We may then pass on to consider the plumed seeds, which possess by far the most efficient as well as the most beautiful devices for aiding dispersal found among wind-borne seeds. These plumed seeds belong to many different groups of plants, and the tufts of delicate hairs which give them their buoyancy arise in different ways. Among the Compositæ, the Order which furnishes the most familiar of our plumed seeds, the plume is formed by modification of the upper part of the calyx, which in so many common plants is small, green, and leaf-like; the lower part of the calyx in the Compositæ is tough, persistent, and close-fitting, forming an additional protection for the seed. The plume springs either from the top of the seed, as in the Thistle, or is borne on a slender stalk, as in the Dandelion. It consists of a ring or radiating mass of hairs of beautiful delicacy, often bearing short
Fig. 13.—Wing-seeds and Plume-seeds.
a, Mountain Willowherb (Epilobium montanum), 2/1; b, Dandelion (Taraxacum officinale), 2/1; c, Mountain Avens (Dryas octopetala), 1/1; d, Scotch Fir (Pinus sylvestris), 2/1; e, Reed-mace (Typha latifolia), 2/1.
branches; these hairs are tightly packed together when the fruit is young or during damp weather, but on a dry day when it is ripe they spread out, and the seed, breaking away from its attachment, is floated off by the wind. In many species the plume or pappus is only lightly attached to the seed, so that if on a voyage an obstacle is encountered the seed drops off, while the now useless parachute drifts away. But though the plume seeds of the Compositæ are the largest and most beautiful among our common plants, they are not the most efficient for dispersal. The fluffy seeds of the Willowherbs (Epilobium) and of the Willows (Salix), for instance, fall at a slower rate than those of almost any Compositæ, while by far the most buoyant seed in the British flora is that of the Reed-mace (Typha). In this case the seed itself is minute, and is situated on a very slender stalk, from near the base of which springs a tuft of delicate hairs. This seed takes thirty-four seconds to fall twelve feet. Using once more the Azorean example, it could cross the 800 miles of sea if it had an initial elevation of 31/3 miles, or was raised to that amount during the sixteen hours occupied by its passage.
Summing up, then, we find that the plume seeds are the most efficient of all seeds for extended flights by the agency of the wind. If the efficiency of the seeds of the Reed-mace, the most buoyant among British plants, be taken as 100, the efficiency of the Willowherbs is between 60 and 70, of Willows 45 to 70, the best of the Thistles 35 to 40, Dandelion 25. Even the best of the winged seeds are much less efficient, Elm and Scotch Fir being about 20, Sycamore and Ash 9 or 10. Of powder seeds, the efficiency of several Orchids tested ranges from 35 to 65, and Broomrapes (Orobanche) from 20 to 25. Most of the powder seeds are far below these, the efficiency of seeds of Papaver dubium, for example, being only 4·5 on the same scale. This last figure is representative of the many small-seeded plants in the British flora such as are found among the Crucifercæ, Caryophyllaceæ, Scrophulariaceæ, etc. The relative efficiency of such comparatively large seeds as those of many of our Leguminous plants would be about 1 on the same scale.
4. Dispersal by Animals.—The coverings of many seeds are provided with hooks or barbs, and others with stiff hairs, which render them liable to become entangled in the hair or fur of passing animals. Examples will occur at once to the reader, as this character occurs in the case of many familiar plants, such as Burdock (Arctium), Enchanter’s Nightshade (Circæa), Avens (Geum), and so on. Without doubt these hooked fruits often secure a wide local dispersal by the aid of cattle, sheep, rabbits, and so on: the state of one’s trousers or stockings after walking the autumn woods is often very suggestive in this regard. Again, herbivorous quadrupeds eat seeds in quantities, many of which are capable of germination after passing through the animal’s body. But while the dispersal obtained by such means may often aid in spreading a species over a tract of land, it does not generally aid in the crossing of barriers, such as mountains or sea, on account of the limitations to the movements of such animals. To arrive at a true estimate of the importance of the animal kingdom in regard to plant migration, we have to study the movements, habits, and food of birds, to whose wanderings neither mountains nor seas set a barrier. Seeds are carried about by birds in two ways—by becoming attached to their feathers or feet, or by being eaten and subsequently ejected. The first case belongs to the class of phenomena which we have just been considering, save that the smooth plumage of birds, and their frequent preening of their feathers, tends to keep their coats free from extraneous material. But at least in wet weather minute seeds must often cling to feathers and to feet, and mud which may contain seeds may easily be present on a bird’s toes during flight. More important is the question of endozoic dispersal—where seeds are transported in the alimentary canal of birds. Some families, like the Finches and Tits, which eat great numbers of seeds, are inimical instead of helpful to dispersal, because the seeds which they devour are crushed and afterwards digested. But in many cases the seeds are swallowed whole, and are usually in no way injured by their passage through a bird’s body. Frequently, indeed, the seeds have not to run the gauntlet of the digestive juices of the alimentary canal, being disgorged from the stomach along with other hard material prior to digestion. Birds which live on berries or other juicy fruits are the most important in seed-dispersal. As Barrows says: “The seed-eaters are not the seed-planters; on the contrary, the insectivorous birds more often sow seeds than the true seed-eaters.” “Seeds which simply contain nourishment are eaten and destroyed, while seeds which are contained in nourishment are eaten and survive.”[5] It is for this reason that, if we look under a tree on which Blackbirds or Thrushes perch, we shall often find young plants of Bramble (Rubus), Ivy (Hedera), Holly (Ilex), or Yew (Taxus). There can be no doubt that birds eat and subsequently eject vast numbers of seeds still capable of germination; many observations and calculations might be quoted. But when we come to apply the facts to the problem of long-distance dispersal, or the passage across serious barriers, we find that important limiting factors must be taken into account. The digestion of birds is remarkably rapid, food being ejected from a half to three hours after being eaten, so that a bird eating seeds and at once flying off in a straight line at, say, 50 miles per hour could not convey seeds more than 150 miles. Secondly, many observations show that on migration birds generally travel with empty stomachs and clean plumage and feet. It is clear, therefore, that, as in the case of wind dispersal, we must look to exceptional circumstances, not normal conditions, to provide opportunities for long journeys on the part of seeds. But for the transfer of seeds from France to England, for instance, or from England to Ireland, it is clear that birds furnish a far more efficient medium than wind or water. In one important particular, dispersal by animals has a great advantage over dispersal by wind—that it is practically independent of the weight of the seeds. Thus, the heaviest of British seeds, the acorn, is carried about by Rooks, just as the hazelnut is scattered by Squirrels, or a head of Burdock fruits by a passing sheep.
Having thus arrived at some idea of the high efficiency for dispersal of many kinds of seeds, it is with some little surprise that we observe—as we may on any country walk—that the plants which arise from these are in general no more abundant or more widely distributed than others which possess seeds devoid of any apparent advantages in this respect—seeds which cannot fly nor float, nor cling to a passing creature, and which are not eaten to any extent by birds so far as observation goes. The truth is, we have to remember, as emphasized in a previous chapter, that the world is already densely populated by plants, all of which survive by reason of their being specially fitted for their several habitats. They have fought in the great struggle for existence, and have established their right to the places which they occupy; they will not readily give way to any newcomer whose seeds happen to be imported into their strongholds. Of course exceptions can be quoted, where plants accidentally or intentionally introduced by man into new areas have not only maintained a foothold, but have spread remarkably. Note the case of the Sweetbrier (Rosa eglanteria) in New Zealand, of the Mexican Bryophyllum calycinum in many Tropical countries, of the American Monkey-flower (Mimulus Langsdorfii) in our own islands; but these are admittedly exceptional. It is nearer the truth to say that the troubles of an immigrant only begin where dispersal ends; and that the chance of seeds carrying out a successful migration is much greater than the chances of their giving rise to a new colony when that migration is successfully accomplished. Every head of the Reed-mace liberates about a quarter of a million seeds of marvellous lightness; yet the Reed-mace does not increase in the country, nor is it a particularly abundant plant even in its chosen habitats. The Foxgloves (Digitalis purpurea) in a wood shed, each plant, say a hundred thousand seeds; yet on an average only one of these attains maturity, otherwise the species would become more abundant in the area. This enormous destruction of seed is largely due to competition. The reception which a plant receives in its new home is the thing that matters, and that may usually be summed up in the phrase “House full.”