224 THE CONDITIONS OF LIFE IN THE SEA [PAKT III 



is a compound which contains carbon capable of further oxidation, 

 and therefore in the synthesis of the latter compound from water 

 and carbon dioxide oxygen must be set free. So we find that 

 a green plant when living in the light, and assimilating the 

 carbon of its medium, gives off elementary oxygen. But it also 

 respires in the same manner as an animal, that is, it takes in 

 oxygen, which burns up some of its tissue substance, giving off 

 carbonic acid. In sunlight the amount of oxygen which is given 

 off in the process of carbon assimilation is greater than the amount 

 of the same element which is used up in the process of respiration. 

 Therefore we say that a green plant gives off a surplus of 

 oxygen to the atmosphere, or to the sea water if it is a marine 

 species. But in the dark, when the stimulus of the radiant 

 energy of sunlight is withdrawn, the process of photo-synthesis 

 ceases and the plant then gives off only carbonic acid, but con- 

 tinues to take up oxygen just as an animal always does. 



The green, red, and brown sea-weeds of the shallow water ; the 

 pelagic algae ; the diatoms ; and the other protophyta of the 

 plankton are the holophytic plants of the sea. But the larger 

 algae are restricted to a comparatively narrow well-lighted zone 

 of sea-bottom near the shore, while the protophyta are present 

 everywhere in the sea where sufficient light penetrates. It is the 

 latter microscopic plants therefore which are responsible for the 

 greater part of the production in the sea. 



Saprophytic organisms. Many of the lower plants grow 

 luxuriantly on soils, or media, containing organic matter, such as 

 sugars or organic acids. These saprophytic plants are represented 

 chiefly by the moulds, yeasts, and fungi. Their food-stuffs are 

 therefore more complex in chemical structure than those of the 

 holophytic plants, and their energy is obtained not so much by 

 the oxidation of the tissues of their bodies as by the fermentation 

 of the materials on which they live. These food-stuffs contain 

 much energy and in the process of fermentation they are de- 

 composed, and a small part only of the energy thus liberated is 

 used up by the saprophyte. A typical case is that of the 

 fermentation of a sugar in the process of brewing. Here the 

 latter substance is decomposed with the formation of alcohol and 

 carbon dioxide, and a part of the energy thus set free is used up 



