INTRODUCTION TO CKYPTOGAMIC BOTANY. 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 greeu, 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 J)lue occurs amongst the lower Algge. The colour of 

 Algae does not require much intensity of light for its develop- 

 ment. Many species of beautiful colours grow at depths 

 where the light must be so small that no Phsenogam 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 Algse and their germs, acts on the 

 atmosphere like the leaves of Phaenogams. But, according 

 to Aim^, as quoted in Payer's Botanique Cryptogamique, 

 p. 17, the colour of Algee 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- 

 latorice disengage noxious gases. Ghara is certainly no 

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

 liarities here. 



78. Algee, 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, Diatomaceie only wMch extend so far. Animals 

 abound towards the South Pole, far beyond the limits of most Algre, 

 and Lichens ascend to greater heights. 



t Fucus vitifolius, for instance, was foimd by Humboldt, 192 feet 

 below the sm-face. It could receive there only half the light of a candle, 

 7 



