DIATOMACEAE. 17 



pass into the resting state in a peculiar manner; they form multicellular bodies, 

 the so-called sclerotia, while the plasmodium retracts its branches and becomes 

 sluggish ; the plasmodium of ' flowers of tan ' forms small tuberous bodies. An 

 important change of structure is connected with this change of form ; the plasmodium 

 separates simultaneously into a large number of polyhedral cells, which are divided 

 from one another by walls of cellulose. A section through the sclerotium shows the 

 latter as fine meshes. When a plasmodium which has thus passed into the resting 

 state comes under the conditions necessary for active life, the cellulose in the walls is 

 dissolved, and the whole body is again changed into a motile plasmodium. It appears 

 then from the above description that the plasmodium proper is an organism, to which 

 the laws of cellular structure do not apply, in other words is a non-cellular body * ; at the 

 same time it has the power under definite conditions of assuming a cellular structure. 

 Apart from their negative geotropism at the time when the fructification is maturing, 

 the plasmodia in their motile condition show sensitiveness to moisture and light 2 . If 

 the tan, in which the plasmodium of the 'flowers of tan' is cultivated, is only 

 moderately moist, the plasmodia make their appearance at once and in large numbers 

 on the surface ; if it is more moist, they show themselves gradually and in the drier 

 spots. If the spot where a plasmodium has appeared is sprinkled with water, the 

 plasmodium disappears, and some time elapses before it reappears. A plasmodium moves 

 away from illuminated spots ; if a stronger light is thrown directly upon these spots, 

 a number of plasmodia collect in them. All these phenomena of motion depend very 

 much on the exact stage in which the plasmodium happens to be, and especially on its 

 being near to or further from the time of formation of fructification. 



II. DIATOMACEAE 1 . 



The Diatomaceae (Bacillariaceae, Brittleworts), form a very distinctly 

 marked group among the Thallophytes, the Desmidieae, a group of the Chloro- 

 phyceae, being the only forms with which they show any points of connection, 

 and these somewhat superficial. They are unicellular plants of microscopic size 4 , 

 and are specially distinguished by the peculiar structure of their membranes. 

 These are strongly silicified and composed of two halves, one of which overlaps 

 the other like the lid of a box. The overlapping edges of the two halves are called 

 the girdles, the two halves themselves the valves. Among the cell-contents are 

 chlorophyll-plates, but the green colour is masked by the presence of a brown 

 colouring matter (diatomin) which is closely allied to the brown colouring matter 

 mixed with the chlorophyll of the Phaeophyceae, and gives the brown or yellow 

 appearance to the coloured portions of the contents of the cells (endochrome-plates) 



1 Sachs, Phys.-med. Ges. Wiirzburg, 23 Nov. 1878. 



2 Baranetzky, Influence de la lumiere sur les plasmodia des Myxomycetes (Mem. de la soc. des 

 sc. nat. de Cherbourg, T. XIX). 



3 Pfitzer, Ueber Bau und Entwicklung der Bacillariaceen (Hanstein, Botan. Abhandl., I. Bd. 2 

 Heft, [also his monograph in Vol. II. of Schenck's Handb. d. Bot.]. Schmitz, UeLer die Auxosporen- 

 bildung der Bacillariaceen (Sitzungsber. der Naturf. Ges. zu Halle, June 9, 1877). [O. Miiller, 

 Gesetz d. Zelltheilungsfolge von Melosira {Orthosira) arenaria, Moore (Mitth. d. Deutsch. bot. 

 Ges. I. 1883), see also Pringsh. Jahrb. XIV. 2, 1883; Id., Die Chromatophoren mar. Bacillariaceen 

 (Deutsch. bot. Ges. 1883). Deby and Kitton, Bibliography of Diatomaceae, London 1882. 

 Schmitz, Fr., Beitr. z. K. d. Chromatophoren (Pringsh. Jahrb. XV. 1884). 



* Freshwater Diatoms, according to Pfitzer, seldom attain a length of mm., and are usually 

 much smaller; some salt water forms are as much as 3 mm. long (Synedra ThallotLrix*}. 



[2] c 



