THE ALGAE 145 



might be supposed, since the same thing is well known to occur in the 

 Angiosperms, where the whole gametophyte generation has certainly been 

 reduced to the gametes only, which therefore appear to be borne upon the 

 sporophyte plant. Fiicus is a parallel case, the gametes being formed pre- 

 cociously from the contents of the sporangia themselves. In other genera of 

 the Phaeophyceae, notably in Laminar ia, stages on the way towards this 

 reduced condition of the gametophyte may be found, the sexual generation 

 consisting of a mere half-dozen cells on which the gametangia are borne, 

 and cases of the precocious germination of zoospores still in the zoosporangium 

 are known. The interpretation of the antheridia and oogonia as double 

 structures is also corroborated by the multiple nature of their walls. 



Traces of the formation of septa in the development of the oospheres and 

 antherozoids have been cited as telling against the above view, because the 

 sporangia of the Phaeophyceae are never septate. If, however, the sexual 

 structures really represent sporangia enclosing a single gametangium, which 

 constitutes the whole gametophyte, then we would expect to find vestiges 

 of septation in their contents, since the gametangia of the lower Phaeophyceae, 

 e.g., Ectocarpus, are definitely multicellular. 



BACILLARIOPHYCEAE 



This group of Algae is markedly distinct from any other and has long 

 been a favourite with naturalists on account of the beauty and symmetry of 

 the sculpturing on the cell walls. Under the old name of " Diatoms " they 

 figure prominently in every book on microscopy, and it is not too much to 

 say that their study contributed greatly to the development of the modern 

 microscope. 



They are entirely unicellular, though the cells sometimes hang together 

 in chains or groups, and the cell wall is of transparent silica, formed in two 

 halves which fit together like a box and lid. On solid surfaces the protoplasm 

 can quit its shell and live naked for a time. The cell contains plastids of a 

 bright yellowish-brown colour, due to a yellow pigment called diatomin, 

 which may, however, be only a form of the fucoxanthin of the Brown Sea- 

 weeds. It is therefore the peculiar structure of the cell wall, rather than the 

 pigmentation, which separates this group from the Phaeophyceae. Apart from 

 diatomin, chlorophyll, carotin and xanthophyll are also present, as in other 

 groups. A fatty oil is formed as the sole product of photosynthesis and the 

 drops are usually conspicuous in the cell, accompanied by grains of volutin. 



The Diatoms are an immense and varied group. They abound in all 

 natural waters, both fresh and salt, and seem to flourish ubiquitously wherever 

 nature presents a damp surface. They can often be seen as a yellow scum 

 on the surface of mud in ditches, while, on the other hand, they are the most 

 important group of plant organisms in the plankton of the sea, that is the 

 population of passively floating microscopic life which fills the surface layers 

 of all the oceans and forms the " pasture " of the fishes. As the siliceous cell 

 6 



