72 
JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
It ia the general rule among Dicotyledons that the vascular bundles 
are common to, and continuous in, leaf and stem, and it is only in a few 
water-plants, viz. Hijyj^uris, Aldrovanda, Ceratoi:)hyllum, and Trapa, 
that, as an exception to this rule, we find a single axial vascular cylinder 
developing constantly at its apex through tho stem, and with which the 
veins of the leaves only become subsequently united.* This axial strand 
is adequate to withstand what little strain there is upon the stem. With 
this reduction of vascular tissue in submerged structures we may class 
the fact that some of the Water Crowfoots present the exceptional 
character of " free " veins in their leaves — veins, that is, which end in the 
cellular tissue without anastomosing with the ultimate branchings of 
other veins. This is a return to a more primitive condition which 
prevails in many Ferns and Gymnosperms. 
Plants in rapidly moving water, such as Filcus, Myrioi)liijllum, some 
Potamogctons and Podostemacece have generally a leathery exterior, such 
as also characterises floating leaves. Nor is it only in internal structure 
that these plants are adapted to the conditions of the medium in which 
they are, for not only, as is well known, do Water Crowfoots in running 
water cease to form any floating leaves and elongate 
those long mosses in the stream, 
as Tennyson styled them, but Mr. Hiern has traced an elaborate mathe- 
matical connection between the forms of their floating leaves — when they 
have any — and the resistance of the current. t 
One other common peculiarity of the vegetative structures of aquatic 
plants, and one the precise significance of which is not, I think, as yet 
known, is their siiminess. In some Algae this excretion, by which we are 
enabled to readily fasten down specimens on paper, would seem to be an 
actual exudation of protoplasmic matter through the walls of the outer- 
most cells ; but in flowering plants it is generally an excretion of 
muoilage by special glands or hairs on the surface. However produced, 
it undoubtedly hinders the diffusion of soluble cell-contents into the- 
surrounding water ; but it is by no means certain that this is the only or 
main cause of its occurrence. 
Having now alluded to the more general characters of the vegetative 
structures in water-plants, we are in a better position to consider the 
question of their origin. In the almost complete absence of evidence 
from fossils, what the nature of the first plants to arise on the earth was 
must be a mainly speculative inquiry. If, however, these may have been 
Myxomycetes, or Fungi, rather than Algte, that is to say, plants destitute 
of green colouring-matter, it is probable that the simplest, and therefore 
theoretically earliest, of the Algae were of a very early date ; nor is there 
any reason to look upon them as derived from fungal types. In this con- 
nection it is noteworthy that Algae of very simple structure are world-wide 
in distribution, and occur in salt, brackish, and fresh waters ; that they all 
contain chlorophyll, and that in many of the lowest types it is not asso- 
* Goebt-1, (JiitUncs of CLassification and S_pccial Morphology, Oxford, 1887, 
p. 4()2. 
t W. r. Hlern, On. lite Forms and. Distrihiitionof the Batracliiinn Section of Ranun- 
odus (tevi:-o(l from the Joiiriuil of Hota'ni,!, 1)S71), pp. "21, 'J'2. 
