General 
ing plants as are usually found growing on the bark 
of trees. 
Similar observations have been made on the Vesuvian 
lavas and on those of Greenland. On these lichens and 
blue-green seaweeds (or alge) make up the first vegeta- 
tion. Mosses are mentioned as characteristic of a later 
stage, but ferns and lycopods are not apparently of 
much importance. 
It was shown, however, that, after the moss or fern 
stage, lavas were at first covered by sparsely scattered 
plants adapted to very dry conditions, and that some 
years elapsed before shrubs and eventually trees could 
grow upon them. 
Now it is generally admitted that the lowest and least 
specialised of all plants are the algze or seaweeds, of 
which the bluish-green family are perhaps the lowest in 
the scale of development. 
The bacteria and other fungi are usually supposed to 
be algze which have lost their green colouring matter. 
Except in this respect, they are on the same very low 
level as the algze. Lichens are compound plants due to 
the union of an alga and a fungus. 
So the first colonisers of the lava were, as one would 
expect, algze and lichens. 
We have no geological data to show whether mosses 
preceded the fern alliance in development, but they are 
surely a lower and less specialised group. 
The flowering plants came upon the world’s stage 
last, and are certainly the most complex of all. They 
form associations of many kinds of plants which both 
co-operate and compete with one another, and amongst 
these associations a wood is the highest, and a thicket 
of shrubs is more complex and better developed 
than an -open scattered growth of small perennial 
plants. 
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