Chap, xxii.] OCEANIC SURFACE PLANTS. 



491 



forms the more immediate progenitors of which lived a terres- 

 trial or littoral existence, and which, having taken to Pelagic 

 habits, have become modified only in less important particulars 

 of their structure to suit their new habits of life. 



The surface water of the open ocean is full of vegetable life. 

 Diatoms are to be gathered in the surface net everywhere, 

 and in high northern and southern latitudes* they abound 

 extremely, so as to colour the ice with their debris, change the 

 tint of the water, fill the towing-net up with slimy masse's, and 

 cover the deep-sea bottom with a silicious deposit of their 

 skeletons. 



In tropical seas, other lowly organized algee especially 

 abound ; mainly OscillatoricE, of the genus Trichodesmium. 

 These algte occur in the water as small brown faggots of 

 minute threads, resembling, as Mr. Berkeley says, minute 

 fragments of chopped hay. Together with these forms others 

 often occur in which the threads are gathered into small 

 globular masses with the ends of the threads all directed 

 outwards. When tracts of the sea are passed through, which 

 are full of this Trichodesmium, the water lighted up by sunlight, 

 when looked down into, appears as if full of small particles of 

 mica, or some such substance, so strongly is the light reflected 

 from the minute bundles of the alga?. 



We met with this alga in greatest abundance in the Arafura 

 Sea, between Torres Straits and the hxw Islands. Here it was 

 at first encountered discolouring the sea-surface in bands and 

 streaks ; as the ship moved farther on it became thicker, and 

 at length the whole sea, far and wide, was discoloured with it. 

 It remained still, however, denser in long streaks, and within 

 these again it was massed in small patches. There was a 

 strong smell from these patches, as from a pond covered with 

 vegetation. So abundant is Trichodesmium in some seas, that 

 one of the explanations of the name of the Red Sea is that the 

 term was derived from the discoloration of the water by vast 

 quantities of Trichodesmiu/ii erythrceiim. 



On the voyage from Ternate to the Philippine Islands, the 

 sea was again seen to be full of minute alga^. In this case 

 there were several other forms beside Trichodesmium, and they 

 were embedded together in small masses of a jelly-like sub- 

 stance, which also contained Diatoms. The water was perfectly 

 full of these masses, and tinted by them of a light brownish 

 colour. 



Besides these smaller alga; living in the open ocean, there 

 are abundance of several species of larger seaweeds which are 

 Pelagic in habit. The Gulf Weed, Sargassum bacciferum, of 



See p. 216. 



