SWAMP. 119 
Where the water is not too deep, and is not flowing, many 
species of seed-plants can grow which are not specially adapted for 
the aquatic life. Communities of this character fall into the two 
classes of swamp and bog. 
Dealing first with swamp, this term is: here used to designate those 
plant-communities which have as their home loose soil, frequently 
consisting of peat—the remains of former generations of plants— 
which is covered throughout the year more or less deeply by water. 
Except forest no New Zealand plant-formation has been so greatly 
modified by man, as will be seen in Chapter X. 
The number of species found in swamps is less than 70, belonging 
to i6 families and 32 genera. Their growth-forms include trees, 
shrubs, lianes, herbs, grass-like plants, rush-like plants, and ferns. 
At least 40 of the species are grass-like or rush-like. Many of the 
swamp-plants grow equally well under much drier conditions, while 
others are confined to swamp. 
The conditions for plant-life differ considerably in different swamps 
according to the amount of nutritive salts they contain, the average 
depth of the water, the nature of the swamp-floor, the degree of 
stagnation of the water, the altitude above sea-level, the latitude 
of the swamp, and the combination of species. 
When dealing with the mangrove (Chapter III), it was shown how 
that tree must provide for a supply of air for its roots. So, too, 
with various swamp-plants, which accordingly have a considerable 
portion of the outer part of their roots made up of a spongy covering 
full of air. This can be well seen in the roots of the New Zealand 
flax (Phormium tenax). This plant also avoids the wet ground, as 
far as it is able, by not sending its roots deep down, but rather 
extending them for long distances close to the surface of the soil. 
It is interesting also to note how the roots of P. tenax when growing 
in dry ground are furnished with an enormous number of root-hairs, 
these alone being capable of absorbing water from the soil. But 
when a root gets down to the actual water, or into very wet soil, 
no root-hairs are produced, but the root then is furnished with an 
outer thin “skin” which readily allows water to pass through it. 
The case of the pukatea (Laurelia novae-zelandiae) may be con- 
sidered here. This fine tree grows in swamp-forest, where the muddy 
substratum is deficient in oxygen. At the base of the tree the great 
plank-buttresses continue as roots which are raised high above the 
ee >? 
