490 SUMMARY OF CURRENT RESEARCHES RELATING, TO 



application to the study of some plankton species. In the first section 

 the author reviews the work and conclusions of other authors, and then 

 describes his own researches on plankton. He finds that the organic 

 part of the membrane is constituted of pectic substauces, to the 

 exclusion of cellulose and callose, a character which distinguishes 

 diatoms from all other plants. A discussion then follows on the rela- 

 tion between the organic and the mineral parts of the membrane. The 

 author finds that the valves of diatoms consist of an organic substance 

 identical with the pectic compounds, which is combined more or less 

 closely with silica. The siliceous skeleton thus formed is impregnated 

 and clothed with an external structureless membrane, which often 

 masks, at least in the plankton species, the characteristic markings, 

 and is rapidly dissolved by reagents. This structure explains certain 

 anomalies observed chiefly in plankton diatoms ; and since pectic sub- 

 stances lend themselves readily to the formation of mucilage, it is easy 

 to explain the abundance which surrounds certain individuals and 

 colonies. 



The views of other authors on the growth of the membrane are 

 discussed and criticised. 



As regards the coloration of diatoms, it is stated that the red of 

 ruthenium and of old aluin-tueinatoxylin have produced the best 

 results. The latter must be prepared five or six months before use, 

 or it must be treated with a certain degree of heat ; otherwise it does 

 not act satisfactorily. The best methods of staining are described for 

 specimens in the fresh state, and specimens after destruction of the 

 contents and dissociation of the organosiliceous substance from the 

 membrane. 



In the final section species of Thai ass iosira, Chsetoceros, Bacteri- 

 astrum, Ditijlium, and Leptoeylindrus are described and figured from 

 the point of view of the membrane. 



Abnormal Coloration of Epiphytic Diatoms.* — J. Comere writes of 

 the abnormal coloration which is found in certain diatoms. In some 

 cases this is a constant character ; in others it seems to result from a 

 decrease in the biological functions of the plants ; and in others again, 

 especially in epiphytic forms, it may be attributed to the special action 

 of the medium and consequently to the particular conditions in which 

 they grow. Instances are given of various species showing differently 

 coloured endochrome. The author then describes diatom material 

 collected by him growing in Oscittatoria, Nymj)hsea, and Ulothrix. He 

 finds as a result of studying these epiphytic species that the modifica- 

 tions which arise in their coloration may possibly be attributed to a 

 supplementary formation of green leucites due to the more nutritive 

 nature of the medium, and this green colour predominates over and 

 masks the usual yellow-brown. Another hypothesis is that a kind of 

 symbiosis may exist between host and epiphyte, since the exact shade of 

 green is always the same in both plants ; and, moreover, the green 

 colour of the epiphytic diatoms is only found when the host-plant is in 

 full vegetation, and does not occur in diatoms which are floating free 

 in the same surroundings. 



* Nuov. Notar., xx. (1909) pp. 1-5. 



