428 E. J. ALLEN. 



in from outside also appear to give somewhat different effects, and, 

 although the experiments have not given sufficiently uniform results to 

 justify a definite statement, I am left with the impression that on the 

 whole samples of water taken from Plymouth Sound, when added to the 

 artificial medium, give better growths than are obtained with samples 

 from the English Channel in the neighbourhood of the Eddystone. 



Now the tanks at the Plymouth Laboratory are worked on a closed 

 system of circulation, the same water being circulated over and over 

 again, so that the principal difference between the water taken from 

 them and that obtained from outside consists in the greater abundance 

 in the tank water of organic compounds, which result from the meta- 

 bolism of living organisms. Is it the presence of some organic substance 

 that is necessary for the growth of the diatoms ? A very large number 

 of experiments have been made with a view to obtaining some hght upon 

 this question, and some of these will now be referred to. 



Ulva infusion. A small piece of green seaweed (about 1 square cm. of 

 Ulva latissima) was boiled for about five minutes in a flask containing 75 c.c. 

 of artificial miqueled sea-water, and was then removed with a sterile 

 platinum needle. In this way a weak organic infusion was obtained. 

 When cold the flask was inoculated with one or two drops of Thalas- 

 siosira from a culture in artificial water. In this organic infusion a good 

 growth was obtained, nearly equal to that in the control in artificial 

 miquel plus 4 per cent of tank water. This experiment w^as repeated a 

 number of times with a similar result. 



Though it is most probable that the result is due to some organic com- 

 pound the experiment is, of course, not conclusive, as an inorganic salt 

 may have been dissolved from the ulva. In any circumstances we 

 obtain no hint as to the nature of the organic substance, and the result 

 remains indefinite. 



It may be pointed out that Miquel * in his account of his original 

 experiments on diatom cultures, insists upon the value of the addition 

 of some organic infusion or maceration to his culture solutions, 



Ulva Extract. A piece of Ulva latissima was washed in several changes 

 of artificial sea-water and then an extract was made in absolute alcohol at 

 a temperature of 58° C. The alcohol was evaporated to dryness on a 

 water-bath. 75 c.c. of artificial miqueled sea-water was then boiled in 

 small portions at a time in the vessel containing the extract, so that all 

 soluble parts of the extract were dissolved. The water was then re- 

 turned to a culture flask, which, when cold, was inoculated with Thalas- 

 * Le Diatomistc, I, 1890-3, p. 95. 



