XXX vi INTRODUCTION. 



circulation is now found in so many other plants as to lead 

 to the inference that it is universal in the vegetable king- 

 dom. It must be remarked, however, that as yet the motion 

 of the sporules has only been observed in certain families, 

 and Mr. Ayardh, in the memoir before us, denies its ex- 

 istence in any but those of the series Chlorospermea, and, 

 with doubt, in Ectocarpus ; whence he proposes to divide 

 the class into two great groups, Zoospermea, having 

 moving spores, and Fucoidece, having motionless spores. 

 But to me it appears that our knowledge of this subject is 

 too much in its infancy to render any such division safe or 

 expedient. Indeed, since the publication of Mr. Agardh's 

 paper, minute bodies, endowed with analogous motion, have 

 been discovered (as stated in a former page) in the antheri- 

 dia of the Fuel, and may be expected to occur in many 

 other Algae. Zoospermeae, as I have before observed, cor- 

 responds to our Chlorospermea, so that, without having 

 recourse to this dubious character, the result of our ar- 

 rangement is the same. 



The relative distribution of plants on the surface of the 

 globe offers a wide and interesting field of inquiry, which 

 has of late years attracted considerable attention from bo- 

 tanists ; and though, as we approach lower and less com- 

 plicated organizations, we find the influence of climate to 

 be less powerful in causing a difference of character among 

 the groups, or even a dissimilarity of species, still among 

 Cryptogamia we may discover evident traces of distinct re- 

 gions of vegetation, analogous to what obtain among phas- 

 nogamous plants. With respect to the Alga, little has been 

 done in this department to follow out and correct the views 

 of Lamouroux, whose excellent essay, published some time 

 after his death in the seventh volume of the ' Annales des 

 Sciences Naturelles,' I shall take as the ground-work of the 

 observations I am about to offer ; but so little comparatively 



