2 DONKINj ON DIATOMACE^. 



in ordinary weather to the influence of breakers^ but only 

 in sheltered quiet nooks, where the tide creeps up and retires 

 again without producing waves, which, beating against the 

 surface of the sand, soon dissipate its tiny occupants. For 

 this reason, the collector may traverse miles of the shore 

 otherwise suitable, without observing a single specimen, 

 until he arrives at some sheltered cove, such, for example, 

 as serves to protect the boats of the fishermen from the 

 violence of the storm. In such favoured spots, the furrows 

 left on the sand by the receding tide will be found to be 

 covered by a chesnut or olive coloured stratum of Diatomaceae, 

 which may be collected in the manner described in my 

 former paper. It is necessary to add, that these habitats 

 should not be ATisited by the collector, except during the con- 

 tinuance of calm weather, as immediately after a stomi all 

 traces of Diatomacese will have disappeared. The third con- 

 dition, however, seems to be as necessary as the other two ; 

 consequently, Diatomacese are only observed on the beach 

 between the latter part of April to the beginning of Sep- 

 tember, as a general rule, when not only the stillness of the 

 sea, but the warmth which the sands acquire from the direct 

 rays of the sun during ebb-tide, favours their propagation. 

 From September to April, the low temperatm'c and the 

 waves of winter prevent their development and aggregation. 



I will only here remark, on the propagation of the Dia- 

 tomacese, that although it has not been shown that they 

 form gonidia, yet I have reason to believe that gonidia, in 

 the form of still or resting spores, are the sources from which 

 the new crop originates on the beach each successive spring. 

 This opinion I have formed from the following facts. First, 

 amongst the myriads of specimens of marine Diatomaceae 

 I have examined in the living state, I have never observed 

 the process of conjugation. Secondly, I have, as a general 

 rule, found the same species luxuriating in the same circum- 

 scribed locality (extending, in many cases, over only a few 

 square yards) which yielded it in the previous summer. The 

 presence of a particular form, year after year, in the same spot, 

 would therefore appear to be due to the propagating cause 

 remaining buried in the sand during the winter, through the 

 course of which not a diatom is to be found. Were the 

 crop of each succeeding spring due to the subdi\ision of a 

 single frustule, or of a few, accidently left by the tides, the 

 same locality would produce, in all probability, widely 

 different forms each returning season. 



I may mention as a fact of some importance, that I have 

 generally found the species most commonly met with on the 



