PLANTS OF THE SEA 



77 



The one place where no photosynthetic plant can exist, of course, is in very 

 dim light or darkness. Because photosynthesis is impossible in such places, one 

 would not expect to find photosynthetic algae below a depth of about 200 feet. 

 Algae become rare below 100 feet. These figures vary according to the nature of 

 the water. 



One group of single-celled animals (Protozoa) is commonly referred to as 

 being more closely related to plants than to animals. This is the zoologists' 

 Flagellata or Mastigophora or the botanists' Euglenophyta. Flagellates are con- 

 sidered by many scientists to be a perfect link between animals and plants 

 since they achieve mobility by means of flagella as do many animals, yet are able 

 to manufacture food by photosynthesis as are most plants. Obviously, according 

 to evolutionary theory, before there were true animals or plants, there must 

 have been some group that gave rise to both and thus possessed the potential 

 attributes of both. Such a group is Flagellata, a group which is considered in 

 the chapter on invertebrates simply because its members look and act more 

 like animals than plants. Flagellates, like small one-celled true plants, are plank- 

 tonic and form a basic food of the sea. Many algae have sexual stages, notablv 

 the male sex cells, that closely resemble protozoan cells. 



Bacteria are very minute and simple plants. They are not true algae. Their 

 very simple order of construction sets them aside from all other plants. They are 

 well known as the causal agents of many diseases and are probably the most 

 common of all living things. They will not be included here, but it must be 

 remembered that they form a basic food for many of the small one-celled animals. 

 Some of them are unique among living things in their ability to use inorganic 

 compounds (nitrates, nitrites, and sulphur compounds) as a source of energy. 

 This method involves no photosynthetic process and indicates the primitive 

 nature of bacteria (deep sea food chains, Chapter 2). 



Many plants of the sea, like garden plants, are known only by their scientific 

 names. Most of the genera described and figured here have several species 

 widely distributed on both coasts. 



DIATOMS: Phylum Ghrysophyta— F/^wr^ 18 



Diatoms are microscopic one-celled algae that live in immense numbers near 

 the surface of the sea, being the most common planktonic plant. They are 

 enclosed in a siliceous shell of great variation and beauty. The shells may be 



,^3«Wj%_ 



Fig. IS. Diatoms. The diatoms are shown much enlarged, the circular one being 

 only about one-fiftieth of an inch in diameter natural size. Drawn from microphoto- 

 graphs hy the authors. 



