INTRODUCTORY. 9 
small] life is given up, and it returns to the earth 
the elemental substances of which it is composed. 
There are, indeed, few parts of the material 
world that are not instinct with the minutest 
forms of vegetable life, though oftentimes their 
presence can only be detected when they exist in 
vast numbers. The surface waters of the ocean 
are sometimes tinged with the most beautiful 
colours by the presence of myriads of almost 
infinitesimal plant forms. Vegetable forms exist 
also in the ocean’s depths. Even in the air, 
and in the very bowels of the earth, it has 
been asserted, by at least one eminent naturalist, 
that plants grow. Plants, too, are to be found 
on the surface of ice-bergs; tinging with 
roseate hue the snow of the arctic regions; 
covering the surfaces of arid plains; crowding 
into the lowest depths of mines; clinging, as 
powdery crusts, to rocks and stones; spread- 
ing, in varying colours—as moulds—over all kinds 
of objects; entering, with subtle and often fatal 
power, as blights, into leaves, stems, roots, and 
tubers; infecting, with a strength like that of an 
epidemic disease, the living organisms of animals ; 
