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 algae) 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 algae 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 algae which have lost their green colouring matter. 

 Except in this respect, they are on the same very low 

 level as the algae. 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, algae 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. 



20 



