196 BOTANY 
Among native hydrophytes the pond-weeds (Pota- 
mogeton) the water-milfoils (Myriophyllum), the water 
starworts (Callitriche), and the floating fern (Azolla) 
are the commonest. In the pond-weeds, the floating 
leaves are oval in shape while those below water are 
more ribbon-like, and thus less likely to sustain damage 
from running water. In the milfoils,the fine thread-like 
leaves are produced in whorls, those below being more 
slender than those above water. The small undivided 
leaves of the starwort are produced in great numbers, 
and cover large areas of ponds and sluggish streams. 
Azolla often forms large masses on the surfaces of 
pools, and in the spaces between its closely-packed 
leaves is the air by means of which the plant is enabled 
to float. 
The chief introduced hydrophyte is the water-cress, 
which is plentiful in streams and ponds, and, like most 
other plants of this society, has its submerged leaves 
more divided than its exposed ones. It is one of those 
water plants that root in the soil, and are most plentiful 
round the edges of a stream or pond where the water 
is shallow. In other places the Canadian water weed 
(EHlodea) is the predominant member of the society. 
Ottelia ovalifolia, a water-plant from Australia, with 
large oval leaves, has now establishd itself in the ponds 
and lakes of the Auckland district. How it first got 
there is not known, but when once established it was 
probably spread by ducks, which carried the seed on 
their feet from lake to lake, 
The author, while in Te Aroha, found that a South 
American water plant, Hydrocleis nympheoides, had 
established itself in the backwaters of the Thames. 
Investigation showed that a settler had placed a number 
of water-plants, including the water-lily and water- 
hyacinth, as well as that already named, in a pond 
formed by an overflow from the Thames, In times of 
flood, some,of these had escaped and established them- 
