THALLOPHYTA— ALGiE 41 



They are both fresh- water and marine in their habit, and are 

 extremely numerous wherever they ocom'. 



Diatoms are unicellnlar plants, of very minute size, which 

 have their cell-walls strongly impregnated with silica, the mark- 

 ings upon the walls often being of great regularity and beauty. 

 The plants may be free, or a number of them may be grouped 

 together. In the latter case the colony may consist of a chain of 

 cells, or they may be aggregated together lapou gelatinous stalks, 

 or may form gelatinous masses. Each diatom consists of a proto- 

 plasmic cell 01 frustule which is encased in two siliciiied shells 

 known as valves, one of which overlaps the other by its edges. 

 The peculiar sculpturing is found upon the flattened face of each 

 valve. The plant is not furnished with cilia, but is nevertheless 

 capable of a peculiar gliding movement through the water, the 

 natu.re of which is at present unexplained. The colouring :natter 

 is deposited in chromatophores of more or less regular form, and 

 is not diffused through the protoplasm as in the preceding gi-oup. 



The diatoms frequently undergo longitudinal fission ; the two 

 valves slightly separate ; the protoplasm divides, and each half 

 secretes a new valve between itself and its fellow, which has its 

 edges included under the rim of the old valve, The two valves 

 of a diatom are thus of different ages. In the free forms this 

 process of fission increases the number of the individuals ; in 

 those which continue attached it recalls the processes of inter- 

 calary growth. 



This repeated bi-partition gradually reduces the size of the 

 diatoms, each new valve being necessarily a little smaller than 

 the one which overlaps it. When a certain limit of size is reached 

 another method of reproduction occurs, which is in some cases a 

 sexual one. This is the formation of auxospores. The contents 

 of two diatoms escape irom their containing valves and unite 

 together, growing into a new plant which secretes a pair of 

 valves like those of the original cells. In some cases the process 

 is asexual ; the contents of a cell escape and grow ; on a con- 

 siderable size being attained, new valves are secreted by the 

 protoplasm and the original appearance is resumed. 



Certain modifications of both these methods occasionally 

 occur. 



Sub-Class III. — Ph^ophyoe.s;, or Melanophyoeas. 



This sub-class includes those Algse which are of an olive- 

 brown or olive-green colour, with the exception of the Diato- 

 macese. Its members comprise forms of very great variety, 



