HALF AN HOUR WITH SEAWEEDS. 35 



may add still further to your list by taking a boat, 

 and dragging the bottom with a clawed drag. Much 

 the same difference occurs with the marine flora as 

 the terrestrial. Height above the sea-level materially 

 influences the distribution of the genera and species 

 on land, and depth does the same with seaweeds below 

 it. Those who wish to study the general laws which 

 regulate this apportionment of animal and vegetable 

 life, are referred to Professor Edward Forbes's theory 

 of the zones of depth which belt every island and 

 mainland. 



Seaweeds are roughly, but sharply, divided into 

 three distinct groups, according to the colour of their 

 spores, which are black (or olive), red, and green. 

 The names given to these are Melanosperms, Ehodo- 

 sperms, and Chlorosperms. Minor, but important 

 subdivisions again occur, based upon the peculiar 

 character of the spores themselves. We will take the 

 black or olive group of seaweeds first, as they are most 

 abundant, as well as strongest and largest. Indeed, 

 these latter qualities enable them completely to mono- 

 polise the ground in many instances, to the complete 

 exclusion of diminutive species, which are forced in 

 self defence often to become parasitic on their more 

 powerful neighbours. In this respect, therefore, 

 their smallness may actually be advantageous to 

 them. We have spoken of such species as parasitic, 

 but this must not be understood in the sense in 

 which we understand parasitism in terrestrial plants. 

 Properly speaking, seaweeds have no real roots, nor 



