THEIR STRUCTURE AND MIGRATIONS. 
23 
species of plants are introduced into the island. Certain kinds of 
seeds will resist the action of sea-water for a very long period. 
Mr. Darwin has shown that about ten per cent, of seeds thrown 
into the sea could be floated by currents across a space of water 
900 miles in width and would then germinate. In Holman’s 
1 Travels ’ an account is given, on the authority of Mr. Keeling, 
who resided for twelve months on Keeling Island, of the various 
seeds and other bodies washed ashore there. He shows that a 
great many seeds are brought from the island of Sumatra and the 
peninsula of Malacca, and that, from the direction of winds and 
currents, they could not have come in a straight line, but must 
have travelled from 1800 to 2400 miles. All the hard seeds, such 
as those of creepers, retained their germinating power, but the softer 
kinds were destroyed in the passage. 
Divers, also, play an important part in the transportation of seeds. 
The huge rafts of matted vegetables and fallen trees which drift 
down the large rivers of the world often convey seeds many 
hundreds of miles. The lightness of many kinds of seeds, too, 
causes them to float for long distances, and after a flood the banks 
are often strewn with them. There has, for instance, been noticed, 
growing not far from the mouth of the Columbia river, one or 
more individuals of Finns ponderosa , a tree which is limited to the 
north and for some distance to the south of this river, to the dry 
region east of the Cascade Mountains. There cannot be a doubt 
that it had been originally brought to the site where it was 
growing in the form of seeds floated down the river from beyond 
the Cascades. Many kinds of seeds and fruits are specially adapted 
for water-transport. The seeds of the water-lily (Nymphcea alba) 
are enclosed in a globular mucilaginous mass of a spongy nature, 
in which there is a number of air-bells which act as floats. As the 
mass disintegrates the air-bubbles which have supported it are set 
free and the seeds escape. These still float until the inflated 
arillus by which they are surrounded becomes waterlogged, when 
they sink. By this provision the seeds can be drifted long distances 
from the parent plant. 
I have hitherto been dealing with what may be called the natural 
means of the dissemination of seeds, but there is another most 
important agency to be considered. Their designed and accidental 
transportation by man influences the flora of the different regions 
of the earth’s surface to a degree almost beyond belief. Wherever 
civilised man takes up his abode there are sure to spring up around 
him some plants belonging to the country in which he last resided. 
Many of our familiar plants have taken a firm hold in New Zealand, 
as, for instance, the watercress, which has been imported and has 
acquired astonishing vigour, filling the rivers and streams to such 
an extent as to render navigation difficult. Our little white clover, 
too, is driving the great New Zealand flax before it. The seeds of 
these plants were introduced, it may be, by some colonist, for 
economic purposes, and they have thriven to such an extent as to 
become a great nuisance. The French gave some New Zealand 
