PLANTS AND ANIMALS 65 



forms living not only in water but in most damp places; 

 (6) Fungi, including toad-stools, moulds, mildews, the microscopic 

 yeast-plants, and the still smaller bacteria; and (c) Lichens, which 

 are intimately connected communities of algae and fungi. 



All these plants, except fungi (and a few seed-plants), contain 

 leaf-green or chlorophyll, a substance of great biological import- 

 ance, as elsewhere explained. It is convenient to distinguish 

 forms which possess it as " green plants", though the chlorophyll 



Fig. 10(34. Delations between Animals and Plants: arrows indicate the taking in or giving out of various substances 



Both green plant and animal take in oxygen (o) and give out carbon dioxide (003) in the course of respira- 

 tion (R). The animal feeds on plants, and by nitrogenous excretion and ultimate death adds to the store of 

 organic matter in the soil. The green plant in the course of feeding (F) takes in carbon dioxide (co 2 ) from the 

 air, returning oxygen (o), and also takes up water with dissolved salts from the soil; its dead parts contribute 

 to the store of organic matter in the soil. The groups of bacteria B^-Bg, respectively produce ammonia com- 

 pounds, convert these into nitrites, and these again into nitrates. The bacteria 64, and the tubercle-fungi Bg, 

 fix the free nitrogen (N) of the air, with production of nitrates. The bacteria B 6 , in the absence of oxygen, 

 decompose organic matter with liberation of free nitrogen (N). 



may be obscured by the presence of other pigments, as, for 

 example, in brow r n and red sea-weeds. 



RELATIONS BETWEEN ORGANISMS AND THE CONSTITUENTS OF 

 THE ATMOSPHERE (fig. 1064). In considering this question it must 

 not be forgotten that the gases which are mixed together in air 

 are also found dissolved in both fresh and salt water, and the 

 relations between these dissolved gases and aquatic organisms are 

 pretty much the same as those subsisting between ordinary air and 

 land organisms. The most important of these gases are carbon 

 dioxide (carbonic acid gas, CO 2 ), oxygen, and nitrogen. It has 

 already been explained in the section on BREATHING (vol. ii, p. 379) 

 that plants and animals respire in the same way, taking in oxygen 

 to facilitate the breaking -down processes which continually go 



VOL. IV. 



99 



