HIDDEN MARRIAGES 



565 



merit like those by which they were produced. Zygnema is common in 

 both running and stagnant water, and forms the green slimy masses 

 frequently seen floating on the surface of ponds and ditches. 



The Desmids are almost exclusively inhabitants of fresh water, and are 

 unicellular. They are quite unattached except temporarily resting, as it 

 were and float freely, having the 

 power of spontaneous motion. Cer- 

 tain species may combine to form 

 threads, but the segments of such 

 threads are separate organisms. 

 They are all very minute, only the 

 very largest of them such as 

 Cosinarium and Micrasterias being 

 just visible to the unassisted eye. 

 They vary greatly in form, but all 

 the members of a genus conform 

 roughly to one type. They are all 

 symmetrical, and each individual 

 is usually divided into two sym- 

 metrical portions by a constriction 

 extending from the margin nearly 

 halfway to the centre. Even the 

 cell contents chlorophyll bodies 

 and starch-grains are symmetric- 

 ally grouped in bands and star- 

 patterns. The transparency of the 

 cell-wall enables these patterns and 

 the rotation of the protoplasm to 

 be seen clearly and distinctly. 

 Some of the long slender species 

 like Closterium and Docidium have 

 clear spaces at their extremities in 

 which " brownian movements " of 

 the particles suspended in the cell- 

 sap may be seen. 



In the large Family DEATO- 

 MACE.E all the species are micro- 

 scopic and abound in water, both 

 fresh and salt. The}^ are all single- 

 celled plants whose cell-wall with few exceptions is highly silicified and 

 takes the form of a slightly unequal pair of valves, each with a rim (girdle), 

 the larger valve overlapping the smaller. The valves are transparent, and 

 enveloped in an outer layer of thin gelatine. These valves are favourite 

 objects with microscopists, who, however, rarely take interest in them as 



FIG. 714. BLADDER WRACK (Fucus vesiculosus). 



Fertile branches bearing conceptacles {con.). 



