MANN] DIATOMS, THE JEWELS OF THE PLANT-WORLD 55 
rather uncommon, being confined to a few species. The third is 
perhaps the most frequent of all and is especially characteristic of 
the moving diatoms. 
Conjugation takes from eight to twenty days for its completion; 
and as the diatoms lose by each act of fission about one-sixtieth in 
length, and as they divide every five or six days under normal condi- 
tions of nourishment, it would require not more than one act of con-- 
jugation yearly to balance the reduction. Frequently conjugation 
does take place only once in a year; and then it is, at least in our 
latitude, quite uniformly early in spring, often before the ice has 
entirely disappeared from the streams. 
Two other methods of reproduction are claimed as taking place 
among the diatoms ; namely, by means of exceedingly minute spores ; 
and by means of daughter plants, two to sixteen in number, formed 
within the body of the parent plant. But as the former is wholly 
unsubstantiated, and the latter, described by G. Murray (in Proc. 
Roy. Soc. Edin., vol. 21) is, to say the least, most anomalous, no at- 
tempt will be here made to describe them. 
There is one other physiological process of the diatoms which has 
up to the present time puzzled all its investigators, their motion. 
Many of the diatoms grow attached to some support, some of the 
round and oval forms lying flat on the fronds of other algz, while 
others are fixed at the ends of gelatinous stalks, singly or in clus- 
ters, or grow as zigzag chains or in rows like beads or in flat bands 
as long filaments. But large numbers are free; and these, especially 
