PLANTS AND ANIMALS 65 
forms living not only in water but in most damp places; 
(4) /ungz, including toad-stools, moulds, mildews, the microscopic 
yeast-plants, and the still smaller bacteria; and (c) Lichens, which 
are intimately connected communities of alge 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 
AIR 
R R 
CO, O co, O 
co / 
FF) ,2 > s 5 
(0 oN NITROG@s 
N \ \ FooP DEATH & DEG, ey Bg 
VUE 
CY: YY WY) YM: / Wi 
Uy ee MAY GLE Ss 
: WL GLI Ni By rai fe 
3 Ye Leth NITRATES LLY // NAT RIT 9 
Yi yy CO YY; iif iy 
Mi MIM OM IMT 
a v4 4 
Gi MM EMMI MUTE 
Fig. 1004.—Relations between Animals and Plants: arrows indicate the taking in or giving out of various substances 
Both green plant and animal take in oxygen (0) and give out carbon dioxide (cog) in the course of respira- 
tion (rR). 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 (co2) from the 
air, returning oxygen (0), 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 8,-B3, respectively produce ammonia com- 
pounds, convert these into nitrites, and these again into nitrates. The bacteria By, and the tubercle-fungi B;, 
fix the free nitrogen (N) of the air, with production of nitrates. The bacteria Bg, 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 brown 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,), 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 
