56 PHYSIOLOGY CHAP. 



a double process, anabolic and katabolic : the first synthetic, re- 

 ducing, assimilatory ; the second analytic, oxidising, disintegrative. 



The antagonism apparent at the extreme limits of function 

 between -the higher plants and animals becomes less and less in 

 proportion as we descend the scale of the two groups of living 

 beings. On comparing the simplest animal and plant organisms, 

 it is impossible to trace a sharp line of demarcation between the 

 two kingdoms. This fact demonstrates their common origin, 

 according to the Unitary Theory of Life, by which plants and 

 animals must be regarded as two divergent stems arising from 

 a common trunk represented by the simplest, or primitive, living 

 forms, to which Haeckel gave the name of Protista. 



The fallacy of this supposed antagonism between the functions 

 of plants and animals lies in a confusion between the katabolic, re- 

 spiratory function, chemically represented by processes of oxidation, 

 which is common to all living beings, and the anabolic, chloro- 

 phyllic function which is peculiar to the green parts of plants. 

 Vegetable protoplasm, including that provided with chlorophyll, 

 breathes like animal protoplasm, i.e. it absorbs oxygen and gives 

 off carbonic acid, when removed from the action of the sun's rays. 

 Under the influence of these rays, it breathes in the reverse sense, 

 i.e. it absorbs carbonic acid and gives off oxygen", because the 

 reducing function of the chlorophyll, which is actively aroused by 

 the soLir radiation, exceeds in its activity the respiration proper, 

 and masks its effects. 



It has long been known that the presence of oxygen is almost 

 always essential to plant as to animal life. As early as 1822, De 

 Saussure was aware that the most vigorous plants, such as the 

 Cactus, die quickly when brought into an atmosphere deprived of 

 oxygen. P. Bert found that wheat germinated less freely in 

 proportion as the oxygen tension of the air in which it was kept 

 was lowered. 



So, too, when tension of carbon dioxide reaches an excessive 

 degree it is as harmful to plant as to animal life. It was, again, 

 De Saussure who demonstrated that plants brought into an 

 atmosphere of C0 2 perished. An atmosphere containing \ of 

 carbonic acid is sufficient to check the germination of most plants ; 

 accordingly, respiration as an oxidative process is a function as 

 indispensable to the life of plants as to animals. 



The antagonism that is sometimes proposed between plants 

 and animals is therefore fallacious, and derives from the fact that 

 the former accumulate the energy elaborated from the sun's rays, 

 while the latter consume it, or transform it into special forms of 

 heat and motion. 



It is in general true that plants cool the surrounding atmo- 

 sphere, while animals raise the temperature ; but this is due to the 

 fact that respiration is not usually very intense in plants, and 



