AQUATIC PLANTS. 
73 
ciated with any other colouring-matter ; that, though often floating just 
below the surface, even when anchored, they pass their whole life in a 
submerged condition ; and, finally, that their chief reproductive structures, 
whether vegetative or sexual, the zoospores and antherozoids, are free- 
swimming ciliated bodies. These characters all point to the Algae being 
primitive aquatics, having, that is, no terrestrial ancestry. The produc- 
tion of ciliated antherozoids, requiring at least a little water in which to 
swim, would point to a similar ancestry for the now largely terrestrial 
Bryophyta and Pteridophyta.* When, however, we turn to flowering 
plants it is doubtful if w^e can find any such primitively aquatic types. 
There are many that vegetate under water, and not a few free-floating 
rootless forms, many too that ripen their fruits under water ; but 
flowering, or the pollination of the flower, nearly always takes place above 
or on the surface ; and, in those cases in which it does not do so, there are 
generally some characters suggesting that the submerged type is the 
modified descendant of some allied land-plants. An aquatic ancestry has 
been suggested for Monocotyledons, and the theory is borne out, not only 
by the simple, glabrous, and entire leaves, so common in the Class, but 
especially by the ribbon-like form so frequent in the cotyledonary leaves. 
The substitution of granular pollen for a ciliated swimming antherozoid 
is, however, a crucial objection, and in most cases other details in 
flowering and pollination point to an ultimately, though possibly remotely, 
terrestrial derivation. No flowering plant is so completely adapted to an 
aquatic life as is the marine Eel-grass, or Grass-wrack (Zostera), now 
known in commerce as a packing material under the name of Alva. 
Widely distributed, the five species of this genus grow with creeping 
stems rooted in the mud on gently sloping shores, and bear long thin 
limp linear ribbon-like leaves, sheathing at their bases. The sheath of 
the uppermost of these leaves serves as spathe to a flattened spadix. 
IThe anthers burst under water discharging the elongated, tubular, one- 
coated pollen, which is carried by the water to the long thread-like and 
also submerged stigmas. These pollen threads are of the same specific 
gravity as the sea-water, so that they float at any depth.f Though 
Delpino has suggested that Zostera is an Aroid adapted to a submerged 
existence, such genera as Posidonia and Buppia connect it with Fota- 
mogeton, and it has been also suggested that the fertilisation may be 
modified from an earlier wind-pollinated condition. In Bii'ppia, however, 
the flowers are at the surface of the water, and the granular pollen floats 
on the surface ; whilst in the allied Zannichellia the pollen is spherical 
in the anther, but becomes tubular w^hen discharged, and performs its 
function under water, much as in Zostera. Similar extreme adaptations 
occur in Naias and Ceratophyllum, pointing to an extremely remote, 
although not primitive, aquatic ancestry in these cases. The extreme 
* Dr. D. H. Scott has the following suggestive passage on the connection between 
the alternation of generations and the change from aquatic to terrestrial life in his 
Structural Botany, part ii. (1896), p. 297 : — " The sporophyte of the higher plants, 
whatever its origin may have been, is specially adapted to the formation of aerial, as 
distinguished from aquatic, spores. The spores of the Archegoniatie, from the lowest 
Eryophyta upwards, differ from those of any of the Algiii in being almost always suited 
for dissemination by the air. The sporophyte which bears them is essentially tlie aerial 
generation, while the oophyte is dependent on water for the act of fertilisation.'' 
f Sciienk, op. cit. ; Kerner, op. cit. i. GCG, ii. 104, 105 ; ^Villis, ojj. cit. ii. o97, 398. 
