Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 2 / Plant-Dispersal

by H. B. Guppy

The next mountain genus I will specially refer to is Lagenophora, one of the Compositæ. The solitary Hawaiian endemic species, L. mauiensis, is restricted to the summit of Mount Eeka, in Maui. In the mountains of Vanua Levu, Fiji, another peculiar species, L. pickeringii, has been found; and there is a species, L. petiolata, in the Kermadec Islands (Hooker, in Journ. Linn. Soc., i. 127); but the genus is chiefly characteristic of Australia, New Zealand, and temperate South America, one species occurring both in Fuegia and Tristan da Cunha. The genus has no pappus; but Hooker in the case of the Kermadec species considered that the “viscid fruit” favoured its dispersal; and this may probably be true of the genus.

With regard to the capacity for dispersal of the seeds of Plantago, it may be pointed out that the seeds of Plantago major, P. lanceolata, &c., become coated with a mucilaginous material when wetted. In 1892, when experimenting on these plants, I found that the wetted seeds adhered firmly to a feather, so that it could be blown about without their becoming detached. Species of Plantago are so characteristic of the “alpine” floras of the summits of lofty mountains in the tropics, as in Java and many other regions, that the mode of dispersal has always been a subject of curiosity. I cannot myself doubt that this is the explanation of the occurrence of the representatives of the genus that now thrive as endemic species on the higher slopes of the Hawaiian mountains. This method of dispersal for Plantago is recognised by recent writers on the subject of seed-dispersal. (In a paper in Science Gossip for September, 1894, I dealt with the “mucous adhesiveness” of such seeds as a factor in dispersal. The subject had previously been discussed by Kerner in one of the earlier volumes of his Pflanzenleben; and I have summed up some of the results in [Note 43] of the present volume.) My readers can readily ascertain by a simple experiment that a bird pecking the fruit-spikes in wet weather would often carry away some of the sticky seeds in its plumage. Several years ago, when I was endeavouring to examine the condition of these seeds in the droppings of a canary, my efforts were defeated by the bird itself, since, in spite of all my care, some seeds and capsules were always carried by the bird on its feathers into the clean cage reserved for the experiment.

The plants of these mountain genera possessing dry seeds or fruits neither very large nor very minute and suitable for bird-food are Ranunculus, Viola, Vicia, Sophora, Artemisia, Sisyrinchium, six in all, or 24 per cent. of the total. On the probable method of transport of the ancestors of these endemic species the following remarks may be made. With regard to Ranunculus, some authors like C. M. Weed (Seed-Travellers, p. 48, Boston, 1899) perceive in the curved or hooked beaks of the achenes a means of attaching the fruit to plumage. This no doubt applies to some species, and it is advocated by Ekstam for some of the plants of the Nova Zembla flora. There are others to which this explanation would not be applicable, and the achenes of the Hawaiian species do not appear to be specially fitted for this mode of transport. I have found the achenes of Ranunculus frequently in the stomachs of birds in England, in partridges frequently, and in wild ducks at times. Those of certain species that possess buoyancy are common in the floating seed-drift of rivers, as of the Thames (Journ. Linn. Soc. Bot., xxix. 333), and they would probably in this way be often swallowed by waterfowl.

I have but few data directly relating to the dispersal of seeds of Viola by birds. From the frequent occurrence of species in alpine floras, as in the Caucasus, the Great Atlas, in the mountains of Equatorial Africa, in Madagascar, &c., it may be inferred that birds transport the seeds between the higher levels of many continental ranges in tropical regions and to the mountain-slopes of neighbouring large islands. Viola abyssinica, for instance, which occurs in Madagascar, is spread over the elevated mountain ranges of tropical Africa. With regard to the five Hawaiian species, it may be remarked that three of them are bog species and two occur in dry situations. The first are most characteristic of the mountains, one species occurring on the summit of Mount Eeka, 6,000 feet above the sea. Judging from the stations alone, at least two species were originally introduced into the Hawaiian Group.

Viola seeds, as indicated by my experiments on the different British species, including Viola palustris, are not buoyant, and there is no possibility of the seeds being picked up by birds in floating drift. There is, however, a possible means of dispersal in birds’ plumage by means of the mucosity of the seeds of some species. Thus, although this is not exhibited, as shown by my experiments, by Viola canina and V. palustris, it is well displayed by the Field-Pansy (V. tricolor). I found that the seeds of this species, after lying a little time in water, were thickly covered with mucus, and that they adhered to a feather, on drying, as firmly as if gummed. This did not, however, come under my notice in the case of the seeds of one of the Hawaiian species, V. chamissoniana, examined by me. One sometimes observes Viola canina in England growing in places, as in the crevices and on the tops of old walls, where its seeds could have only been carried by birds. In some cases the propellent force of the seed ejected by the contracting valves of the capsule would explain queer stations. In its power of seed-expulsion, Viola chamissoniana, the common Hawaiian species, is just as active as our British species.

With regard to the Leguminous genus Vicia we have the observation of Focke on the dispersal of its seeds by pigeons, as described before on page [150].

Sophora chrysophylla, the “Mamani” of the natives and one of the most familiar of the trees of the Hawaiian mountains, is discussed at length in [Chapter XV.], where the difficulty of supposing that its seeds could be transported unharmed in a bird’s stomach half-way across the Pacific is pointed out; and it is suggested that it was more probably derived from a littoral species brought by the currents. However, the point is a debatable one, and the seeds of the “Mamani” can scarcely be regarded as “impossible” from the standpoint of dispersal.

With reference to the possibilities of dispersal of the achenes of Artemisia, some very suggestive indications are to be obtained from a paper by Mr. D. Douglas on the North American Tetraonidæ published in the Transactions of the Linnæan Society for 1833. The “Cock of the Plains” (Tetrao urophasianus), as we here learn, makes its nest on the ground under the shade of Artemisia bushes, and lives on the foliage and fruits of these and other plants. This bird is plentiful in Columbia and North California, and another allied species is mentioned which lives on the same sort of food. Later authors refer to these and other birds of the same family as living chiefly on the Sage-brush (Artemisia tridentata), a plant prevailing over great regions of the plains as well as on the slopes of the Sierra Nevada and of the Rocky Mountains. According to Dr. Sernander (page [228]), birds when feeding on the fruits of Artemisia vulgaris in the district of Upsala scatter them about and thus aid in its dispersal. Artemisia achenes, since they have neither pappus nor other appendages, nor any special adhesiveness when wetted, depend largely on their small size and light weight to aid them in dispersal. (Those of A. absinthium measure a millimetre in length, or ¹⁄₂₅ of an inch, whilst those of A. vulgaris measure 1·8 mm., or ¹⁄₁₄ of an inch.) Driven as we are to look to bird-dispersal for the means of transport of Artemisia achenes, it is interesting to find a possible source of the Hawaiian endemic species on the nearest American mainland, even though it is some 2,000 miles away. It is assumed that they would be ordinarily carried in adherent soil or entangled in the feathers, and on rare occasions in the bird’s stomach.

The small seeds of Sisyrinchium possess no means of adherence to plumage. They are crustaceous, and in cases where the stomach and intestines of a bird are well filled with other food they are quite capable of resisting injury. The solitary Hawaiian species has, according to Hillebrand, a range in altitude from 3,500 to 7,000 feet. I found this pretty herb most abundant on the “cattle-plains” of Hawaii between 5,000 and 6,000 feet, where it is evidently in part dispersed by the cattle and other animals. The seeds are very small, being about a millimetre in size, and when dried nearly 100 go to a grain (0·65 decigramme). They might thus also be transported in mud on birds’ feet.

For the mode of dispersal of the minute seeds of Lobelia, the last of the mountain genera to be specially noticed, I must refer the reader to the remarks on this subject in [Chapter XXII.] They would probably be carried in soil adhering to the legs or feet of a bird.