THE STRUGGLE FOR EXISTENCE 197 
selves as stated. Fortunately the current of the Thames 
is too strong for the water-hyacinth, or we might have 
our rivers and lakes choked with this pest, as has been 
the case in Florida and Queensland. So long, however, 
as this plant is in cultivation there is always the danger 
that through birds or other means it may get into 
favourable surroundings and become a source of serious 
trouble. 
LD. Cockayne, Ph.D., F.R.S., photo 
Flax (Phormium tenax) swamp near New Brighton, Canterbury, 
being invaded by willows and European grasses. 
Swamp plants, as well as water plants, are included 
among the hydrophytes. Their chief adaptation lies in 
the fact that their lower parts, which are buried in the 
mud, are suited to a life in water, while their aérial parts 
as a rule resemble those of ordinary land plants. 
The commonest native swamp plants are the raupo 
(Typha) and the flax (Phormium), the latter requiring 
less water than the former. The spongy leaf of the 
raupo with its numerous air-spaces is characteristic of 
the true hydrophyte. The flax leaf, however, has a 
