36 INTRODUCTION. 



which dry up and disappear at the approach of summer, the 

 species inhabiting them having performed their allotted office 

 in the economy of nature, disappearing likewise, their lives 

 terminating with the exigency which called them into ex- 

 istence ; others are usually met with in the perennial waters 

 of our deep and clear lakes and ponds, and, though not them- 

 selves perennial, yet have probably a life of somewhat 

 longer duration extended to them ; as, for example, some of 

 the larger Zygnemata, certain species of Draparnaldia 

 and Lyngbya and Conf. crispata. I am inclined to think, 

 however, that the lives of but few species of freshwater 

 Algce extend beyond the period of a year, while it is very 

 certain that very many perish in a few months, or even 

 weeks, from the time of birth, in which case I can assert 

 from observation, that the species perishing thus early are 

 frequently reproduced in the course of the summer, when 

 the circumstances are favourable, some two or three times. 

 Very many Conferva die in the spring from the drying up 

 of the waters in which they dwell, at which season it is 

 wisely ordained that such species should mature their seeds ; 

 amongst these may be mentioned many species of the genera 

 Zygnema, Tyndaridea and Vaucheria ; others die at the ap- 

 proach of winter, but not all; a few linger through the 

 greater part of this season so unfavourable to the exercise of 

 the vital functions of plants. There are other species, again, 

 which do not require to be constantly immersed in water, 

 but are found upon those soils and in situations which retain 

 moisture for some time, as upon shaded and clayey pathways, 

 at the roots of trees, on banks, thatch, and at the bottom of 

 palings, the drippings from which they receive. In such lo- 

 calities, Conf. ericetorum, some Lyngbyece, Scytonemea, many 

 OscillatoricB and Nostochinece are met with. In these plants, 

 the strength of the vital principle must be very great, for 

 their filaments may be dried up for a considerable time ; but 

 on the application of moisture, they soon recover their 

 healthy appearance. 



So abundant are the productions under our consideration, 

 that there is not a ditch or pool of any extent or standing 



