104 THE VICTORIAN NATURALIST. 



smaller than their predecessors, so that a diminution in diameter 

 becomes obvious ; but sometimes the new valves are a little 

 larger, thus accounting for the progressive increase in size observ- 

 able in the case of the filamentous species already noticed ; while, 

 in the free forms, frustules of different sizes may be met with, the 

 larger being the more numerous, the increase in number having 

 taken place in geometrical progression, while that of size was 

 uniform. Instructive illustrations of the foregoing in the case of 

 Biddvlphia may be found upon some slides representing a 

 gathering of diatoms I obtained from Corio Bay last Easter. 



Reproduction in the true sense — i.e., preceded by conjugation, 

 and resulting in the fertilization of sporules or gonads, which, 

 after a period of repose or incubation, enclosed within a cyst, 

 attain a condition of living and independent life, and resembling 

 in every respect the parent cell — we have not time to touch upon. 

 I can only refer you for information to the recent works of 

 specialists in this particular branch of study, as those of Count 

 Castracane, O'Mera, and others. 



I have been at some little pains to show, in as few words as 

 possible, that the auxospore theory as applied to the whole family 

 of diatoms is not accepted by eminent diatomists, although it has 

 found its way into nearly every botanical text book, since it is not, 

 as already mentioned, founded upon or supported by actual 

 observation ; and, further, to show that even in the matter of a 

 knowledge of the reproductive process, supposed to have been set 

 at rest years ago, we have yet very much to learn. 



Not less interesting than the problems connected with repro- 

 duction are those belonging to the power of spontaneous move- 

 ment possessed by some genera. Everyone who has examined 

 living diatoms, particularly the Navicula and allied forms, must 

 have noticed the peculiar jerking movement with which they are 

 endowed. In the case of the Pleurosigma, and some other genera, 

 this movement is smooth and equable, and proceeds for about 

 the length of the frustule and then back again. If any small 

 obstacle is met with in its course it is usually pushed aside; if, 

 however, the object is too large, the frustule is detained for a 

 length of time equal to what it would have taken if it had com- 

 pleted its forward course, and then retires from the impediment 

 as if it had not been so detained. By Mr. W. Smith this power 

 of movement is referred to forces operating within the frustule, 

 and originating in the vital operations of growth, &c. 



Light and heat are also the causes of certain movements — more 

 particularly, I think, the latter. Confirmatory of this, I may 

 mention that I have in my possession at the present time a 

 gathering of Pleurosigma decorum obtained from the sea, from a 

 depth of three fathoms or so. They were found about the middle of 



