INTRODUCTION TO CETPTOGAMIC BOTANY. 197 



in the open sea, are sufficiently abundant. Melobesia occurs as 

 high as 74° N. lat. ; there are, however, many genera which affect 

 warmer seas, especially those of Australia, and they abound iu 

 the tropics. Gorallina officinalis, as its name implies, is ad- 

 mitted into the Pharmacopeia on account of its calcareous 

 nature, but there is no peculiar virtue in it which makes it 

 superior to other similar chemical matters. Dr. Johnston be- 

 lieved the Melobesiw to be mere states of GoralliTM officinalis. 



174. I am not aware whether corallines are grazed upon 

 by fish, as coral is by the genus Sparus. Mr. Darwin has de- 

 scribed the manner in which chalk is formed in the latter case, 

 and, from the large quantity of carbonate of lime which these 

 plants contain, it is highly probable that they contribute their 

 share to its formation.* 



13. LaurenciacejE, Harv. 



Cyliadrical or compressed, cartilaginous. Conceptacles ovate, 

 with a termiaal pore. Spores pear-shaped, radiating from a 

 basal placenta. Tetraspores lodged in the branchlets. 



175. These agree with the next tribe, Rhodomelacece, in 

 fruit ; but the frond iu the greater number of species is inarti- 

 culate and solid, though sometimes tubular and septate, with the 

 superficial cells minute, and not forming evident reticulations ; 

 the tetraspores too are scattered over the branchlets. The 

 antheridia are highly developed, and assume curious forms, 

 in those cases where they have been observed. Nothing can 

 be more variable than the colour, even iu the same species. 

 Laurencia affords some of our commonest Algae, and it is weU 

 known to every collector how these vary. When perfect they 

 are of a dull purple or brownish red ; but these tints pass 

 through every shade to orange, and yellow, and green, 

 according to the degree of their exposure, in shallow pools, 

 or open rocks, to the action of light and air, while fresh 

 water is rapidly destructive. Others, like Asparagopsis, are 

 of a beautiful pink; while ChaTnpia assumes the hues of 

 Polysiphonia, exhibiting, like many other Algae, prismatic 

 colours. The species are very widely distributed, occurring in 

 very different locahties. Champia, as originally understood, 



* Darwin, Journal, p. 553. 



