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—z.. 
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 voluine 
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 
raphe, 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- 
