INTllODUCTION TO CRYPTOGAMIC EOTANY. 97 



limits, such variations are indicative of, or at least accompany 

 divisional characters. Some possess a beautiful herbaceous 

 green, others exhibit the bright hues of flowers, and there 

 are few tints which are not displayed by individual species. 

 Next to green, however, the most prevailing tint is pink, 

 passing into various shades of purple ; or, on the other hand, 

 olive, from a bright tawny or golden green to black. Even 

 pure blue occurs amongst the lower Algas. The colour of 

 Alga? does not require much intensity of light for its develop- 

 ment. Many species of beautifid colours grow at depths 

 where the light must be so small that no Phasnogam could 

 exhibit anything of its proper hue, supposing it possible for 

 its blossoms to be developed under such conditions. 



77. It has been long known that the green matter of Priestley, 

 consisting of the lower Alga? and their germs, acts on the 

 atmosphere like the leaves of Phaenogams. But, according 

 to Aim4 as quoted in Payer's Botanique Cryptogamique, 

 p. 17, the colour of Alg£E is, in this respect, indifferent; and 

 marine Algae in general absorb carbonic acid, and disengage 

 oxygen, under the influence of the sun, exactly as Phaenogams. 

 The quantity of oxygen disengaged by them is said to be im- 

 mense. M. Aim^ collected nearly two pints from a space of 

 about two and a half square yards, by agitating the fronds. I 

 believe, however, it will be found that some of the Oscil- 

 latoricG disengage noxious gases. Chara is certainly no 

 Alga, and, therefore, I have no reason to speak of its jiecu- 

 liarities here. 



78. Algaj, in some of their varied forms, occur in all parts of 

 the globe, reaching the utmost confines of vegetation, and, are, 

 perhaps, capable of flourishing under greater extremes of 

 temperature than any other organised beings.* Within certain 

 depths which, however, descend in some cases to very 

 numerous fathoms t they abound in both fresh and salt water, 



* It is, however, Diatomace(e only which extend so far. Animals 

 abound towards the South Pole, far beyond tlie limits of most Alo-fp, 

 and Lichens ascend to greater heights. 



t Fumis vitifolius, for instance, was found by Humboldt, 192 feet 

 below the surface. It could receive there only half the light of a candle 



7 



