[Chap. XLl NON-GREEN PLANTS 513 



External digestion and the digestion of substances that cannot be 

 digested by most other organisms are outstanding features of this group 

 of plants. Among the animals there are a few protozoans, such as those 

 in the alimentary tract of termites, that can digest cellulose. External 

 digestion, however, is a common characteristic of animals, for the food 

 within the alimentary tract is still outside the cells of the body. The proc- 

 esses that occur in the digestion of bread in the alimentary tract of an 

 animal are no different fundamentally from those that occur when bread 

 is decomposed by mold. The plant may digest certain substances the 

 animal cannot, but saprophytic plants are present in the alimentary tracts 

 of animals and they may digest certain substances there. Some wood- 

 boring beetles can chew the wood, but their internal fungi digest it. 



Without the saprophytic plants the surface of the earth would soon 

 become encumbered with the undigested remains of plants and animals. 



'^DISSOLVINGN 



GREEN 

 PLANTS 



SUGAR 



\NIMAL$ 



AND 



PARASITIC 

 PLANTS 



'/? 



PLANT 



AND 



ANIMAL 

 .RESIDUES/ 



LIMESTONE 



/ORGANISMSN 



sDther Rocksy 



Y % 



:arbon dioxide 



/OCEAN WATER\ 



AND k^ 



VOTHER NATURALS 

 WATERS 



CARBON 

 DIOXIDE 



OF THE 



V ATMOSPHERE; 

 CO2 



t)EPOSITION^ 



OF 



ARBONATESl 



I as Shells. Marl, 

 Vand Coral Rsefa 



,v<^ 





r^Oli, 



■-£2!^SUST10N 



DECAY 



iacteria, FunpV 



COAL, 

 fPETROLEUM'l 



AND 



GAS 



Fig. 228. The caibon cycle in nature. 



Except in bogs where the growth of saprophytes is comparatively slow, 

 only a thin surface layer of humus has accumulated in all of geological 



