106
��FORMS AND DISTRIBUTION OF BATIIACHIUM.
��tenellus, 1.
tcrrestris, 8-19, 22, 35, 13-16.
trichodes, 25, 28, 29.
tridiopliyllus, 25, 23, 23-34.
trilobus, 15.
trinacrius, 15.
��tripartitus, 8, 10, 7, 12, 4. triphyllus, 12. trisectus, 35. tnincatus, 16. villosum, 25. vulgaris, 17-19, 11, 16.
��Tlie ultimate co-ordinate forms recognized in the previous notes must not be considered as generally equivalent to sub-species; in many cases, at least, their characters depend upon purely external or accidental causes and the plants are not genetically distinct. Thus the peculiarity of the form cgespitosus is manifestly due to the dryness of its habitat, whilst the differences between several forms with ttoating leaves and the correspond- ing ones without tioating leaves are not likely to be of even sub-si)ecitic value. On the other hand, some forms retain their characters throusjh very diverse circun)stances, and they can, for many purposes, be regarded as true species; indeed, it is only by taking a wide and comprehensive view of the group in general, or by comparing some cases of approach on. the part of others that certain of the forms would appear at all likely to belong to the same aggregate species. Some little has been done by growiiig the plants from seed towards showing that certain forms are possibly derived from others ; but nnu;h more is required to prove that all the forms can be so altered by cultivation or otherwise ; and moreover, it is quite likely that no amount of cultivation or manipulation could, in any reasonable time, complete the proof that all the forms are interchange- able by descent. \Vilh the object of clearing up many doubts, it is very desirable that further experiments should be made to test their capacity for variation due to age, climate, season, kind of water and strength of current, descent, and other circumstances ; that more specimens should be brought from those parts of the world that at present have yielded few and isolated forms ; and that, whenever any person meets with a state ditterent from the well-known forms, specimens should be forwarded to some botanist who is well acquainted with the plants and takes a special interest in them.
For complete synonymy the ante-Linnean names should be quoted ; but as these consist in many cases of long phrases, and have quite dropped out of common use, it has been thought better not to encumber these notes with so much extra matter as the bare enumeration of them would involve, and that for so little practical utility. It is interesting, however, to know that the old botanists, from the time of Dioscorides downwards, recognized and named several of the forms ; indeed, about seventy- five ante-Linnean names are extant. They relate to what have latterly been called R. hederaceiis, L., R. aqnatUis, L., R. heterophylliis, Web., R. pdtatus, Schrank, R.fwniculaceus, Gilib., R. pectinatus, Dubois, R. paHcistamineus, Tausch, and to the forms heder(pfolms, homoiopliyUus, fiorlbundm, circinatm, trichophyllus ^\\(\ fiuitans of this enumeration.
The floating leaves in the hederaceus and heterophyllus groups, when developed under favourable circumstances, have in many cases curved out- lines of much beauty and regularity ; it is thus seen that such curves obviously obey definite and exact laws, and inquiry is naturally suggested into their nature.
The following results arc taken from a paper read by me on March 13th,
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