210 THE MICROSCOPE. 



Algce , or marine sea- weeds, are usually classed by 

 botanists in three great groups, each of which contains 

 several families : these are again divided into genera ; and 

 these, in their turn, are made up of one or more species. 

 The species found on the British coasts number about 380. 

 They are grouped into 105 genera. We must not enter 

 into the niceties of classification, but confine ourselves to 

 their general features. Taken in the order in which they 

 present themselves to us on the shore, and limiting each 

 by its most obvious characteristic, that of colour, we may 

 observe, that the group of green sea-weeds (Chloro- 

 spermece) abound near high- water mark, and in shallow 

 tide-pools within the tidal limit ; that the olive-coloured 

 (Melanospermece) cover all exposed rocks, feebly com- 

 mencing at the margin of high-water mark, and increasing 

 in luxuriance with increasing depth, but that the majority 

 of them cease to grow soon after they reach a depth which 

 is never laid bare to the influence of the atmosphere; on 

 the contrary, the red sea- weeds (Ehodospermece) gradually 

 increase in numbers and in purity of colour as they recede 

 from high- water mark, and are never subjected to great 

 changes of light or temperature. 



Dr. Harvey l writes of algce : " Some are so exceedingly 

 minute as to be wholly invisible, except in masses, to the 

 naked eye, and require the highest powers of our micro- 

 scope to ascertain their form or structure. 



" Others, growing in the depths of the great Pacific 

 Ocean, have stems which exceed in length (though not 

 in diameter) the trunks of the tallest forest- trees ; and 

 others have leaves that rival in expansion those of the 

 palm. 



" Some are simple globules or spheres, consisting of a 

 single cellule, or little bag of tissue, filled with a colouring 

 matter ; some are mere strings of such cellules, cohering 

 by their ends, as in Mesogloia, fig. 121; others, a little 

 more perfect, exhibit the appearances of branched threads ; 

 in others, again, the branches and stems are compound, 

 consisting of several such threads joined together, and in 

 others, the tissue expands into broad flat fronds, 



" Only the higher tribes show any distinction with 



(1) See Dr. Harvey's British Marine Algce, or Pfiyc. Britan. 



