64 DEPARTAIEXT OF THE yAVAL HERYICE 



panied by Sl-eletonema cosiaium, Chaetocems dehile and Melosirfx hyp'er'boreu, singly 

 or in combination. All forms except Melosira lost their vitality in a few weeks. One 

 culture was swamped by Nitzschia closterium and another by Chaetoceras sp. ?, a small, 

 delicate form, found singly or in pairs, of which the compressed frustules were 

 rectangular in zonal view and furnished with delicate setae. The others became 

 exhausted. The initial mixed growth obtained in this position showed a decided 

 superiority to that of position 1, but it lasted not more than a week longer, and the 

 final development of Nitzschia and Melosira was superior in position 1. In position 

 3, no growth was obtained. 



It is noticeable that although Chaetoceras dehile was one of the dominant plankton 

 forms it developed in but one culture and then to but a slight extent. Another series 

 started on July 11 from plankton in which Oh. dehile was even more abundant gave 

 a similar result. Two other species of Chaetoceras also, diadema and laciniosum, were 

 more abundant than Skeletonema costatum but gave much less growth. Although 

 chains of all three could be found for several weeks with the aid of a microscope no 

 visible growth was obtained. 



In considering the results from the three positions it may be said that the majority 

 of forms thrive best in strong, diffuse light, but that some forms, Nitzschia and 

 Melosira, are uninjured by direct sunlight. Subsequent work, however, has shown that 

 even they are unable to persist, when the light is too intense. The majority of plankton 

 forms show also an aversion to crowding and die out after a slight increase. This 

 may be due to the exhaustion of some essential nutrient and suggests that interesting 

 developments may be met along such lines; or the exhaustion may be due to the 

 influence of the products of metabolism. The power of living in a crowded area is 

 decidedly greater in some forms than in others, among the least persistent being the 

 members of the genus Chaetoceras. 



A mixed culture maintained during the autumn gave a very considerable develop- 

 ment of several species. Sl-eletonema costatum was the prevailing form, but Aster- 

 ionella japonica was remarkably abundant and healthy. One colony was seen to 

 cantain eighty-five frustules, and the colonies were numerous. This culture was started 

 on August 25, and its position was changed several times before it was finally placed on 

 September 9 in a permanent position opiwsite to a bright, south window. Until the 

 latter date little development was noted, but later it continued to increase until 

 Xovember 13, when the maximum development was obtained. Chaetoceras sociale, 

 Nitzschia hilohata, Coscinodiscus suhhidUens and Thalassiothrix nitzschioides were 

 present in numbers. 



Pure Cultures. 



In position 2 (a window receiving no sun) one very luxuriant, pure culture of 

 Thalassiosira nordensMoldii was obtained. By a pure culture is intended one quite 

 free from other organisms. It reached its maximum in two weeks and remained in 

 excellent condition until the end of July, when the chains began to break up. When 

 at its height the water was filled with a brown cloud of suspended chains. Several 

 flasks were inoculated from this, and for some time showed excellent growth; but after 

 the middle of August its vitality seemed lost, for no further cultures could be started 

 from the original and those already started rapidly deteriorated, so that by August 25 

 very few healthy frustules could be found. The original was retained but showed no 

 subsequent revival. • 



Later a pure culture of Skeletonemn costatum was obtained in addition to those of 

 Nitzschia closterium, and Melosira hyperhorea already mentioned. Only the two latter 

 forms, however, proved persistent. In all cases, except where a culture was swamped 

 by the development of Nitzschia, or some navicular form, the most profuse growth was 

 obtained in the pure cultures. In these, at the end of two weeks, diatoms were present 

 in such numbers that the water was visibly filled with clouds of their chains. 



