Page:Hardwicke's Science-Gossip - Volume 1.pdf/23

] such common occurrence. It may, however, be more common than is generally supposed, as it is likely to be overlooked.

It is more especially with respect to the last two species that our remarks will henceforth apply. After a little observation and practice in distinguishing these two plants in the water, it will be noticed that the Gibbous Duckweed has a more luxuriant appearance, a brighter green colour, and a more polished surface than its lesser neighbour. It will be seen to display a more convex face, and its habitat is quite different, for whilst the Lesser Duckweed can be found only in tolerably clear water, the Gibbous Duckweed affects those localities where one might expect to find typhus fever and cholera morbus rampant. The amateur in search of the latter species must look for a pestilent and loathsome ditch, rich with the putrid remains of dead dogs and defunct cats. In such a place the Gibbous Duckweed grows and fattens, for, be it noted, this is probably the sole cause of its gibbosity. If the said ditch is followed towards one of its extremities, where the water may be purer, it will certainly be found that the Gibbous soon becomes mixed with the Lesser, and, at last, in the pure element the Lesser Duckweed reigns supreme. The above circumstances first became known to us by accident. Knowing the Gibbous Duckweed to be rare, and the "rarity" having at length been found, three or four hundred fronds were collected, and placed in an aquarium, covered with a flat glass to induce flowering to take place, without the least effect. In the course of three or four days, a botanical friend was shown the plants of the Gibbous Duckweed, and lo! they were no longer gibba, but were all minor. The experiment was repeated time after time, whenever the Gibbous Duckweed was taken from its unclean element, and placed in purer water, it immediately put the Banting process into operation, and got rid of its superfluous fat, ultimately becoming a Lesser Duckweed. This would tend to show that these two species are one and the same, the particular habitats causing the species to become either bloated or thin. It is necessary to see how far this idea can stand the test of investigation. The only trustworthy difference between the two species is in the ovules; for whilst in the gibbous form, two or more are said to be produced in each ovary, those of the Lesser Duckweed produce but one. The difficulty, therefore, is reduced to this,—can a plant, which has normally a one-seeded ovary, occasionally bear two ovules or seeds? In both species the ovules are probably always suspended, as in the Lesser Duckweed (fig. 7), whilst in the Ivy-leaved Duckweed the ovules are erect. We think that it is in the experience of every botanist that a one-seeded ovary, under certain circumstances, will bear two ovules; and, if so, where is the line of demarcation between these two species? The only way to decide it would be to reduce the gibbous form in pure water, and force it to flower, if possible, and see if, under the altered conditions of the fronds, one ovule or two would be produced.

A very economical aquarium may be extemporized for these little plants in a tumbler, a fish-globe, in fact any open glass vessel, and there will always be something to study, often something to learn. The mode of growth of the fronds is very interesting, and can be seen best in the Ivy-leaved Duckweed; an enlarged figure, drawn from nature, is given (fig. 13).

Fig. 13. Fronds of Ivy-leaved Duckweed (enlarged), showing mode of growth. Fig. 14. Section of frond of the same species.

If a single frond be taken, it will be found that nearly the only part solid throughout is where the midrib would be, if the frond were like a leaf. The leaf-like expansion on either side consists of two plates, one at the top, and one at the bottom, with a fissure between them (as shown in the section, fig. 14). Within this fissure, on either side, grow secondary fronds, each with similar lateral fissures, from