IV.] ALGM. 361 



It follows fherefore that the Lichen may be regarded as 

 a composite structure, made up of Alga and ascomycetous 

 Fungus, each element capable of at least independent 

 asexual propagation, but incapable alone of reproducing 

 a Lichen which necessarily requires the concourse of both. 



Several species, as Lecanora and Roccella, afford a valuable 

 purple and mauve dye ; and a few are edible, as the so- 

 called Iceland Moss {Cetrarta t'slandica). The Reindeer 

 Moss {Cladonia rangiferind) is a lichen extremely abundant 

 in polar regions, serving as food to the reindeer. One or 

 two species of Parmelia growing upon rocks in Southern 

 India are used in medicine. 



7. Natural Order, Algcs, — The Sea-Weed Family. 



This Family includes an enormous number of species, 

 nearly all adapted to grow under water, though by no means 

 all marine, as many are wholly confined to fresh water. 

 They vary to an extraordinary extent in size, form, and 

 mode of reproduction. Some are microscopic and indi- 

 vidually invisible to the naked eye ; whilst others, especially 

 some marine species, attain a large size — a i&w indeed, 

 measuring some hundreds of feet in length. The simplest 

 forms consist of single microscopic cells : hence they are 

 called Unicellular Algse. These multiply by division, and 

 also by a kind of sexual reproduction, analogous to that of the 

 higher plants, in which the contents of two distinct individual 

 cells become commingled, and the resulting mass finally 

 resolves itself into a number of new individual cells or plants. 



The same species generally present also a sexual process 

 of reproduction, varying in complexity from the simplest 

 condition, in which two accidentally contiguous, but pre- 

 cisely similar, cells of the same or different filaments mingle 

 their contents and so form a germ-spore, giving origin to 



