66 JOUEXAL OF THE ROYAL HORTICULTURAL SOCIETY. 
while most British species of aquatics range throughout the North 
Temperate Zone, and the rapid increase of the Water- cress (Nasturtium 
officinale) in the waters of New Zealand is well known, even the sub- 
tropical genus Azolla will often escape from our indoor tanks and, though 
susceptible to frost, spread for a time in exposed waters. The remarkable 
tropical order Podostemacece (dicotyledons resembling mosses or liver- 
worts and adapted to life in rapid streams, the systematic position of 
which, as is common in such cases, is most uncertain) has been cited * as 
an exception to the usual rule of wide distribution in the case of aquatics ; 
but as the genera Podostemon and Tristicha extend from Brazil to 
Madagascar and South Africa, and the former also occurs in North 
America, only the species can be termed local. In tropical waters this 
uniformity of temperature will result in uninterrupted vegetative growth 
all the year round, and vegetative growth and multiplication, as opposed 
to sexual reproduction, appears to be a general characteristic of aquatic 
plants. 
In temperate waters hibernation becomes a necessity, since almost all 
aquatics are perennial, and, as such, would be liable at the surface of the 
water to be destroyed by frost. In the whole series of aquatics Marsilia 
— one of the Hydropterideae— iV^^ms and Suhularia are almost the only 
annuals, the generally perennial habit being perhaps itself the result of 
the encouragement of vegetative growth by a uniform temperature. 
Hibernation is carried out in a variety of ways. Buppia, Zannichellia, 
Callitriche, and Ceratophyllum, for example, remain unaltered, the last 
named sinking to the bottom in autumn and rising again in spring. A 
second group, including the Water-lilies (NymphcBacecs), Potamogeton 
natans, and others, form more or less fleshy or starchy rhizomes in the 
mud and die down to them in autumn, like ordinary terrestrial herb- 
aceous perennials. A third group, such st,s Potamogeton pectinatusf and 
Sagittaria form tuber-like branches. In the Arrov/-head {Sagittaria 
sagittifolia, L.) these " renewal shoots," as they have been termed, are 
short branches, formed in the axils of the leaves, 
which burrow into the mud and swell up at the ends each into a large bud whose 
central axis is swollen with reserve materials. In spring this develops into a new 
plant. J 
This type nearly approaches some of the fourth group, a large series 
that form special winter buds, generally large buds with closely packed 
leaves which drop off and sink to the bottom in autumn, rising again in 
spring. Such buds are formed at the ends of horizontal stolons in the 
Frog-bit (Hydrocharis Morsus-rance, L.). Kerner gives § an interesting 
figure of Potamogeton crispus, L., which, he says, 
produces late in autumn, near the surface of the water, shoots possessing short leaves 
which are detached from the old stem before the uppermost layer of water is frozen. 
These sink . . . and bore their way into the mud by their pointed lower extremities. 
* J. C. Willis, Manual and Dictionary of the Floivering Plants and Ferns (1897), 
vol. ii. p. 302, a work to which I am mostly deeply indebted for information and 
suggestion. 
t Schenk, Die Wassergewcichse, Bonn (188G), p. 8G. 
J Willis, op. cit. ii. 335. 
§ Kerner von Marilaun, Na'ural History of Plants (Eng. trans.), vol. i. p. 551. 
