FLOATING PLANTS 121 



possess large, winglike outgrowths or frills round the 

 horizontal furrow, etc. (Figs. 38, 39, 41). The majority 

 of the Peridiniales possess a yellow-brown pigment 

 very similar to that of the Diatoms. The colouring 

 matter is confined to small bodies, which lie in the 

 living contents of the plant. 



The Peridiniales reproduce themselves by division 

 into two. In some cases the two new individuals do 

 not separate from one another, but divide further, 

 with the result that chains are produced, as in 

 Ceratium. 



Many of the Peridiniales are capable of emitting 

 light, and the luminosity of the colder waters is mainly 

 due to these forms. 



Pyrocystales. In this small family we find an in- 

 teresting form Pyrocystis which owes its name to 

 the fact that under certain conditions it is brightly 

 luminous. The diffused luminosity of the sea in 

 warmer regions is largely due to this organism. Pyro- 

 cystis is found in most tropical and subtropical 

 regions where the temperature is above 68 F. There 

 are two chief forms P. pseudonoctiluca and P. fusi- 

 formis. The former is spherical and about 0-6 to 0-8 

 millimetres diameter. It possesses an outer trans- 

 parent coat of cellulose, and the internal contents are 

 tinged brown (Fig. 45). 



P. fusiformis has a similar distribution to that of 

 the other species, but has an elongated, boat-shaped 

 form. 



Green Algae. There is in the open sea only one 

 common member of the ordinary green algae, which are 

 so common on the land. This is a beautiful plant with 

 a globular body, known as Halosphaera viridis, found 

 in the warm and temperate waters of the Atlantic and 

 Mediterranean. It has an outer transparent wall, 



