392 K. .1. AI.LKN AND K. W. NEI,S()N. 



form of the frustnles tends to come Ijack to tlie normal again, 

 when the culture is well started, and in old sta<:^es the 

 majority will be perfectly formed, although small and pale. 

 It was found that the addition of silica (in early experiments 

 as fragments of potassium silicate) was, as far as could be 

 judged, immaterial, which fact led to the conclusion that a 

 sufficiency dissolved out from the glass flasks in which the 

 cultures wei-e kept. During rapid growth, it is possible that 

 the silica does not dissolve out fast enough to supply the 

 demand, although it is also ])ossible that diatoms, during rapid 

 division, cannot absorb silica and form a perfect skeleton, 

 even when the supply is abundant. Kichter (18) has proved 

 the necessity of either CaSi.iO; or KoSi^Oj for the growth of 

 Nitzschia pa lea, grown in pure cultures. We tried the 

 addition of silica in various forms, and in one instance, in a 

 cultui*e of Coscinodiscus excentricus, to which a little 

 pi-ecipitated calcium silicate had been added, the uniformity 

 and markings of the valves were much more regular than in 

 the control. The presence of a trace of pnre, dialysed silica 

 also, in one experiment, gave an improved regularity of form, 

 but the quantity or rapidity of growtli did not seem to be 

 affected. No sign of regeneration could be obtained in 

 exhausted cultures by the addition of silica. 



Organic Infusions. — Miquel recommends the use in 

 culture media of infusions of organic substances, such as 

 bran, straw, diatom broth, etc., in addition to the saline solu- 

 tion. He does not make it quite clear if he ever dispensed 

 with them at all. In his general directions he certainly 

 states that the addition of both saline and organic nutrient 

 material is necessary. As would be expected from the general 

 metabolism of plants, the saline constituents are sufficient for 

 growth. At the same time, excellent cultures have been 

 obtained from dilute organic infusions, both with and without 

 the addition of Miquel's solutions A and B. About a square 

 inch of Ulva was boiled in GOOc.c.of sea-water for half-an-hour, 

 cooled, and filtered. In this medium an excellent growth of 

 Coscinodiscus excentricus in one case, and Biddulphia 



