18 SEAWEEDS 
them, but there appears to be little doubt of their 
plant nature. Mixed with these also in temperate 
seas are the Coccospheres (Fig. 580),and inhabiting the 
warmer seas of the tropics the Rhabdospheres (Fig. 
58a), organisms of highly probable plant nature, but 
less studied even than the Peridiniew. Their broken- 
down parts—known to geologists as Coccoliths and 
Rhabdoliths—are, like the remains of Diatomacee, 
known from the chalk, and now play an important 
part in laying down the deep-sea deposits of non- 
polar seas, associated in this (as also in life) with the 
animal Foraminifera of the globigerina oozes. Min- 
gled with these organisms there is a profusion of 
pelagic Protophyta, which sometimes, as in the case of 
Trichodesmium erythreum, form great banks dis- 
colouring the ocean over large areas, and in their 
origin resembling the fresh-water phenomena known 
as the “breaking of the meres” in Shropshire, and 
described by de Candolle and others as occurring in 
the Lake of Morat and other places, by which large 
sheets of water are tinged green or reddish owing to 
the colossal multiplication of minute fresh-water 
Algze. Such occurrences have been often noted in 
the ocean, and, though ordinarily inconspicuous, the 
Alge that cause them, and other allied forms, are 
always present in considerable numbers, as disclosed 
by the use of the tow-net. Other organisms of 
abundant occurrence in blue water are Pyrocystis 
noctiluca (Fig. 57), a source of the brilliant luminosity 
of tropical seas; Halosphwra viridis (Fig. 55), of a 
wider range in warm and temperate seas ; and other 
Protococcacee. The investigation of this pelagic flora 
