INTRODUCTION TO CRYPTOGAMIC BOTANY. 123 



Thomson. In the United States, a few species occur, and are 

 described by Bailey, and probably the number of these will be 

 much augmented. Arthrodesmus taenia, Ehrb., is recorded 

 amongst others. They occur princii)ally where there is some ad- 

 mixture of peat, and in clear pools rather than in running streams, 

 and never where the water is muddy. In limestone countries, 

 where the soil is merely alluvial, they are comparatively rare. 

 This is, however, true, only of the more noble species. Such 

 genera as Scenedesmus are, perhaps, more common in limestone 

 districts, than elsewhere, and scarcely a glass of water can be set 

 aside, exposed to the influence of light, which does not produce 

 a host of forms or species. A few species are preserved in a 

 fossil state, in flints and other transparent minerals. It is diffi- 

 cult, at least in comparing recent and fossil specimens, to deny 

 their identity, so precisely similar are they in form. They are, 

 however, principally spores, and not, as Ehrenberg supposes, 

 perfect forms of Xantliidium. Several species are described by 

 Mr. H. H. White, in the Microscopic Journal, vol. ii. p. 35. 

 Baily has detected cells of various species of Clostervuin and 

 Euastrum, in fossil marls of New Hampshire and New York, 

 and in the marl of Scotchtown, N. Y., below the Mastodon 

 giganteus. Spores have been found by Mantell in the grey 

 chalk of Folkestone, 



98. Much controversy has existed with respect to their true 

 nature ; but at the present day, few advocates will rank on the 

 side of Ehrenberg ; for if in some points there be anomalies, 

 as in Closteriuni, their whole history is so evidently vegetable, 

 their mode of increase, growth, &c., that if we refuse them the 

 title of vegetables, we may as well dispute that of the whole 

 tribe of zoosporous Algse. As for their producing occasionally 

 bodies, endowed with active motion, it is now a matter of cer- 

 tainty, that such bodies exist in a variety of Algag of very 

 different construction ; and it is by no means a fact without 

 exception, that membranes, unless cleansed from every extra- 

 neous matter, will not exhibit the reaction peculiar to cellulose, 

 on the application of the proper chemical tests. But, perhaps, 

 the most important point of all, is the fact that, under the in- 

 fluence of light, they give out oxygen, which, added to other 



