INTRODUCTION. 15 



Algse is far from being understood. A few years back I paid 

 considerable attention to the subject, but without arriving 

 at any satisfactory conclusion. The Staurocarpus ccerulescens 

 is not uncommon near Penzance, is generally in large quan- 

 tity where it occurs, and, from its peculiar colour, cannot 

 escape detection ; on these accounts I made it a principal 

 subject of my observation. Although I have yearly gathered 

 it in several pools, and the sporangia are always abundantly 

 produced, I have particularly noticed during five or six 

 years' observation, that it has never in a single instance re- 

 appeared in the same pool. At Dolgelley, where also in some 

 years it is common, Imet with the same result, with a single 

 exception when I gathered it in one pool for two successive 

 years. I have noticed the same fact with regard to Zygnema 

 curvatum, and I believe it holds good in regard to most if 

 not all the other Conjugatse ; but as they are more liable to 

 be overlooked, I cannot speak of them with the same cer- 

 tainty as of the above-mentioned species. Algae in running 

 water commonly recur every season. I called Mr. Jenner's 

 attention to the subject : we were alike unsuccessful in our 

 attempts to ascertain the cause of this singularity. His 

 observations in general agreed with mine, that the plant will 

 not appear in the same pool for two successive years ; but 

 he found too many exceptions to justify any certain con- 

 clusion. 



All the Desmidiese are gelatinous. In some the mucus is 

 condensed into a distinct and well-defined hyaline sheath or 

 covering, as in Didymoprium Grevillii and Staurastrum tu- 

 midum ; in others it is more attenuated, and the fact that it 

 forms a covering, is discerned only by its preventing the 

 contact of the coloured cells. In general its quantity is 

 merely sufficient to hold the fronds together in a kind of 

 filmy cloud, which is dispersed by the slightest touch. When 

 they are left exposed by the evaporation of the water, this 

 mucus becomes denser, and is apparently secreted in larger 

 quantities to protect them from the effects of drought. I 

 have observed more especially that Tetmemorus granulatus 



