INTRODUCTION. 11 



Mr. Jenner, who informs me that the covering of the spo- 

 rangium swells, and a mucus is secreted, in which minute 

 fronds appear and, hy their increase, at length rupture the 

 attenuated covering*. That some purpose distinct from 

 that performed by the zoospores is served by the coupling of 

 the cells and formation of the sporangium cannot be doubted : 

 for, where we can trace the operations of nature, we find that 

 nothing is useless or in vain ; nor is it reasonable to suppose 

 that this complicated process should fulfil no other purpose 

 than one already provided for without it. The sporangia are 

 most abundant in spring before the pools dry up ; and I 

 would suggest, as no improbable conjecture, that the zoo- 

 spores may be gemmae, analogous to those present in Mar- 

 chantia polymorpha and Lunularia vulgaris, and that they 

 possess merely a limited vitality, which is destroyed unless 

 they are at once placed in circumstances favourable to their 

 growth, whilst on the other hand, in the conjugated cells 

 some important change takes place during the commingling 

 of their contents and the formation of the sporangium, like 

 what happens in the production of seeds in general, which 

 renders the sporangia capable of retaining the vital principle 

 uninjured throughout long periods of drought. 



That the Closteria couple and produce sporangia, in a 

 manner similar to the Conjugatse, has been recorded by Turpin 

 and other writers. Correct figures of some species in that 

 state are given by Ehrenberg in his ' Infusoria,' and Mene- 

 ghini mentions thatBrebisson had detected it inDesmidiumf, 

 but I am not aware that the conjugation of other Desmidiese 

 was Jkdciced by any writer before I published in the ' Annals 

 of Natural History' full descriptions of the formation of 

 sporangia in Tetmemorus granulatus and Staurastrum mucro- 

 natum. Subsequently examples have been detected in almost 

 every genus, and we cannot hesitate to consider it charac- 



* An example of this condition occurring in Closterium acerosum is figured 

 in the Plate containing that plant. 



t " Diatomatum more secedunt, hasque simul e latere copulari in speciebus 

 nonnullis detexit cl. Brebisson." — Meneghini, Synop. Desmid, p. 203. 



