INTRODUCTION TO CRYPTOGAMIC BOTANY. 



83 



I know of no external distinction,* except their place of growth 

 and difference of colour. The definitions of the three groups, 

 as given by Agardh and Brongniart, will not hold good in 

 numberless instances. The definitions given above, will, at 

 least, point out the main distinctions ; but it is probable that 

 the rooting threads of Botrydium (Fig. 24), Caulerpa, and 

 of many of the calcareous Algaj, do absorb nutriment from 



Fig. 24. 

 Botrydium Gramdatum, magnified. 



the soil, and, perhaps, for the reason that they are frequently 

 exposed to the dry air, and would, therefore, wither without 

 such a provision, or are so incrusted with lime as to render 

 nutriment through the surface precarious. It can scarcely be 

 supposed that Lichens never derive any nourishment from 

 their matrix, because certain species grow only upon certain 

 kinds of rock, and some burrow into the matrix ; still, even in 

 these cases, it is clear that the principal part of their nutriment 

 is not imbibed by the spongy tufts, or scattered filaments on 

 the under side of the thallus. 



* There are, indeed, recondite differences, as will appear hereafter, 

 some of which have only lately been ascertained. 



6 * 



