GENERAL INTRODUCTION 25 



selves by " spores " instead of seeds produced by flowers. 

 We may divide them into two types, fixed and drifting. 

 The former are the sea weeds and may be further divided 

 into four groups each of which has a characteristic colour 

 as well as structure. There are (i) Blue-green Algae (Cyano- 

 phyceae), (2) the Green Algae (Chlorophyceae), (3) the Brown 

 Algae (Phaeophyceae), and (4) the Red Algae (Rhodophyceae) ; 

 these are found from high-water mark downward into deep 

 water in the order named, ; this is discussed in more detail 

 later. The first-named are of slight importance, they are 

 minute plants which sometimes form a slimy film over rocks, 

 but many of the others are of considerable size, the Brown 

 Algae including, not only the strange floating Sargassum 

 weed which gives its name to the Sargasso Sea in the Gulf of 

 Mexico, but also the largest known plant in Macrocysiis 

 pyrifera with fronds 200 yards long which is found off 

 the southern parts of South America. Since plants demand 

 a certain minimum of light, all trace of weed disappears 

 from the bed of the ocean below a certain depth. The deep- 

 est water from which weeds have been taken with any cer- 

 tainty that they were actually growing on the bottom 

 appears to be about 60 fathoms, but normally they are not 

 found abundantly except in shallow water. 



The drifting plant life, apart from the large Sargassum 

 weed, consists of minute single-celled plants called diatoms 

 which have tiny silica cases, and also the Peridinians which, 

 as stated above, can be considered either plants or 

 animals as they have certain characteristics of both. Both 

 occur in untold billions and are of primary importance in 

 the economy of marine life. 



