(C) There is also another general rule, and it is this:—Seeds or fruits that float for a long time in sea-water usually float almost as long in fresh water. Here belong the greater number of buoyant seeds and fruits, those only able to float for a few weeks being comparatively few. Now with the long-floating seeds and fruits, those for instance that float in the drift of English rivers from the autumn to the spring, or those that are transported by currents over the tropical zone, there is, as a rule, but a slight difference between their flotation periods in fresh water and sea-water. If one of them sinks after floating for several months in fresh water, it will sink in sea-water a few days after. Fruits of Scævola Kœnigii, pyrenes of Morinda citrifolia, and seeds of Thespesia populnea, Ipomœa grandiflora, Cæsalpinia bonducella, and of different species of Mucuna, that had been kept afloat for a year in sea-water, floated just as buoyantly in fresh water at the close; and in those cases where any sank during the course of the experiment, it was ascertained that they were able to float in fresh water almost to the end.
That many of the seeds and fruits of tropical littoral plants that are known to be dispersed by the ocean-currents will float well in fresh water is shown in the constant occurrence in the floating drift of Fijian estuaries, where the water may be quite fresh or brackish, of the seeds and fruits of plants like Cerbera odollam, Clerodendron inerme, Entada scandens, Heritiera littoralis, Ipomœa pes capræ, Morinda citrifolia, Mucuna, Vigna lutea, &c. In the same way I noticed afloat in the Guayaquil River in Ecuador, when the water was quite fresh, seeds and fruits characteristic of the sea-drift, such as those of Anona paludosa (seeds), Entada scandens, Ipomœa, Mucuna, Vigna, &c.; and when we supplement observation with experiment, as for instance in the case of Anona paludosa, we find that they will float equally long in fresh and sea-water.
The same rule prevails with most of the buoyant seeds and seedvessels of plants of the British flora—seeds and fruits, as I may remind the reader, that are mostly to be found in river and pond drift. I am not able to distinguish any difference of importance in the results of the separate fresh-water and sea-water experiments. Thus with the seeds or seedvessels of Bidens cernua, several species of Carex, Galium palustre, Iris pseudacorus, Lycopus europæus, Ranunculus repens, and numerous others, the difference after a flotation of many months was but slight. If the results of the separate experiments were to be compared, there would be at least ninety afloat in fresh water for every hundred afloat in sea-water; and if at the end of a sea-water experiment, whether occupying three, six, or twelve months, the seed or fruits were to be placed in fresh water, quite nine-tenths and sometimes more would remain afloat. A striking illustration of the principle that the excess in density of sea-water, as compared with fresh water, adds but little to the floating capacity of seeds is to be found in the results given in [Note 41] of simultaneous experiments made some years since by Mr. Millett and myself at Marazion and in London on the seeds of Convolvulus soldanella.
(D) In their relation, therefore, to the density of fresh water and sea-water, most seeds and seedvessels may be placed in two principal classes, the first including quite four-fifths of the total, where they are much heavier than sea-water, and the second comprising most of the remainder, where they are much lighter than fresh water.
(E) It would be surprising, however, if there were not some seeds or seedvessels that come between these two extreme groups; some, indeed, that have a specific weight approximating to that of fresh water, or to that of sea-water, or fluctuating between them, and presenting such evidence of a fine adjustment that the observer, forgetting that they are members of a series, might be apt to regard them as specially adaptive in their origin. It will thus be seen that this subject is gradually assuming a statistical character; and in truth we shall ultimately recognise here the play of the laws of numbers.
As an example of the plants where the specific weight of the seeds or fruits is near that of fresh water, Alisma plantago may be taken. In the course of an experiment, by lowering the density of the water from 1·025 to 1·020, I sent a shower of floating carpels to the bottom. The results vary considerably, as one might expect; but, generally, during the first few days of an experiment about twice as many (sometimes in all as much as 80 per cent.) sank in fresh water as in sea-water, a few only floating in either water for long periods.... The seeds of Arenaria peploides present an example where the specific weight is between that of fresh water and of sea-water. For the purposes of dispersal they may be considered as heavier than fresh water and lighter than sea-water. The details are given in [Note 18]; but it may be remarked here that plants possessing seeds or fruits that sink in fresh water and float in sea-water are very rare. As indicated below, this is what we might look for on statistical grounds.
Plants whose seeds or fruits are not much lighter than sea-water are exceptional. In such cases the effect of increased density of the water is to extend the period of flotation. Thus, in my experiments on the nutlets of Scirpus maritimus, the majority of the fruits floated in fresh water only eight to ten days; whilst in ordinary sea-water they floated in most cases two to three weeks; but when the density was raised to 1·050, the greater number of them were afloat after two months. In a few plants, as with Spiræa ulmaria, the effect of the difference in density between fresh and sea-water was not to extend the period of flotation, but to increase the number that floated for a given period, the extreme limit of the buoyancy of the carpels in either water with this species being about three weeks.
Amongst tropical plants, as illustrated by those of the Pacific islands, cases also came under my notice where the mean specific weight of the seed is somewhere between those of fresh water and sea-water. The seeds of Afzelia bijuga, an inland as well as a littoral tree in Fiji, offer an interesting example. If we place 100 seeds of a littoral tree in sea-water, we find that on the average about 70 float. If then we lower the density gradually, some of the seeds begin to sink at once; and on the removal of the survivors to fresh water, about 47 will remain afloat. The results may thus be stated:—Out of 100 littoral seeds, 30 are specifically heavier than sea-water (1·025); 23 are between sea-water and fresh water in specific weight; whilst 47 are lighter than fresh water (1·000). When, however, we take 100 seeds of inland trees, we find that on the average 87 are heavier than sea-water, 5 are in weight between sea-water and fresh water, and 8 are lighter than fresh water. The significance of these figures becomes evident when we arrange them in curves. The combined result for littoral and inland seeds is given in the diagram below; and we see there, what is also indicated with the separate curves that we are dealing with a double series, one concerned with seeds lighter than fresh water, and the other with seeds heavier than sea-water. The reader can himself supply the separate curves for the littoral and inland seeds. The point, however, to notice is that if a botanist with a statistical bent were to make a miscellaneous collection of the seeds of the Vesi (Afzelia bijuga) in one of the Fijian islands, in order to test their buoyancy, he would obtain such a result as is given in this diagram. Two varieties of the tree would be at once indicated, and further research would indicate that these varieties were connected with littoral and inland stations. This subject is further dealt with in [Chapter XVII.]
Combined results for 200 seeds of Afzelia bijuga (100 littoral; 100 inland).
| Percentage. | Heavier than sea-water, or +1·025. | Between sea-water and fresh water in weight. | Lighter than fresh water, or -1·000. |
|---|---|---|---|
| 100 |  | ||
| 80 | |||
| 60 | |||
| 40 | |||
| 20 | |||
| 0 | |||