190 SEAWEEDS 



reach so great a size as the marine, the former rarely 

 attaining one third of a micro-millimetre in length, 

 while some marine forms approach three micro- 

 millimetres. They may be distinguished in the 

 mass by their brownish colour. The infinitely varied 

 markings on the siliceous valves afford systematic 

 characters, and are fully displayed after cleaning i.e. 

 the removal of the organic matter by calcination, or 

 by treatment with nitric acid. Owing to the silicified 

 membranes, the plants when once formed are incap- 

 able of further growth, though alterations of volume 

 occur by the sliding of one valve over the other to the 

 extent of the breadth of the girdle. It is impossible 

 by mere verbal description to convey an adequate 

 idea of the variety and beauty of the forms assumed 

 by Diatoms, or of the fine sculpturing of their valves. 

 In describing them it is the practice to denote the 

 aspect in which the girdle side is presented as the 

 girdle or zonal view, and the aspect which displays 

 the surface of the valve as the valve view. When 

 the latter is presented the central portion is fre- 

 quently free from fine markings, and may be tra- 

 versed longitudinally by a line called the suture or 

 rapJie, while nodes or nodules representing thickenings 

 occur at each end, or in the centre as well. 



Each Diatom contains a nucleus, and the colouring- 

 matter occurs in plates or bands of more or less 

 regular form ; drops of oil are frequently also pre- 

 sent. A resting condition sometimes occurs, in 

 which pairs of new valves are formed within the 

 original ones. Such states are called craticular 

 states, and they appear to correspond to those con- 



