38 INTRODUCTION. 



pieces, and yet each separated portion will retain its vitality 

 unimpaired, and go on increasing as before; but this is 

 owing, in a measure, also to the fact of each cell in the 

 series enjoying an independent vitality. They also sustain, 

 unharmed, considerable vicissitudes of weather, notwithstand- 

 ing which they are, however, regular barometers, rising and 

 sinking in the fluid medium which surrounds them alter- 

 nately, either as the sun shines, and warmth is diffused, or as 

 clouds and rain obscure the sky, and cold prevails. In this 

 way, too, they protect themselves in a great measure from 

 the alternations of weather, the water being much warmer 

 beneath the surface than on it. This power which the 

 Conferva possess of rising and sinking in water, in cor- 

 respondence with atmospheric changes, is to be explained by 

 reference to their specific gravity, which is in proportion to 

 the activity with which the function of respiration is carried 

 on. During the autumnal and early spring months, the 

 Conferva remain almost entirely at the bottom of the water, 

 except when tempted by a few sunny days to rise to the 

 surface, and expose themselves to the contact of the air, so 

 that the naturalist engaged in the investigation of these 

 productions is often surprised on visiting ponds in which he 

 beheld the day previously Conferva floating on the surface 

 in considerable quantity, to find that on his next visit they 

 have all vanished. The Conferva are also amongst the first, 

 if not the very first, subjects of creation to feel the approach 

 of more genial weather, beginning to vegetate sometimes so 

 early as the months of January or February. 



In the preceding pages it has been assumed that the fresh- 

 water Alga are really what many observers have been 

 inclined to doubt, viz. vegetable productions or plants, they 

 being led so to do, first, from the curious and extraordinary 

 motions of the zoospores already described, and second, from 

 the peculiar and animal fo3tor which the different species 

 exhale during decomposition. Their true position in the 

 scale of organized beings has been, it seems to me, satis- 

 factorily determined, not merely by reference to certain re- 

 semblances which they bear to vegetables in appearance and 



