BALANCE-SHEET OF NUTRITION. 109 



water and carbon dioxide are the most important ; and this is 

 the origin of the carbon dioxide given off. It will appear here- 

 after that precisely the same action takes place in the respira- 

 tion of animals, and that all living things breathe or respire in 

 essentially the same way. 



It was for a long time believed that a leading difference between plants 

 and animals lay in the fact that the former give off oxygen and absorb 

 carbon dioxide, while the latter give off carbon dioxide and absorb oxygen. 

 But it is now known that both give off carbon dioxide and require oxygen, 

 and that only the chlorophyll-bearing parts of green plants are endowed 

 with the special function of decomposing carbon dioxide and water, and 

 manufacturing starch as a result of which they do (but in the light only) 

 give off oxygen. 



PTERIS AQUILINA. 

 (Balance-Sheet of Nutrition.) 



INCOME. 



Matter. 

 Food. 



Inorganic salts, 

 Carbon dioxide, 

 Water, 

 Free oxygen. 



Energy. 



Sunlight absorbed by chlorophyll, 

 Heat. 

 (Potential energy in foods?) 



OUTGO. 



Matter. 



Carbon dioxide, 



Water, 



Excreted substances, 



Reproductive germs, 



Leaves, etc., 



Free oxygen from decomposition of 



carbon dioxide in light. 

 Energy. 



Work performed, 



Heat, 



Potential energy in cast-off matters, 



reproductive germs, etc. 



Balance in favor of Pteris : 

 Matter. 



Tissues, protoplasm, starch, cellulose, chlorophyll, etc. 

 Energy. 



Potential energy in organic matters. 



Physiology of the Tissue-Systems. The epidermal tissues serve 

 as the sole medium of exchange between the inner parts of the 

 plant and the environment ; they are protective, and in certain 

 regions are useful for support. The function of reproduction 

 also falls upon these tissues, as is shown by the development of 

 the sporangia, antheridia, and archegonia. 



The jlbro-vascular tissues serve in part as a supporting skele- 

 ton, for which functions their richness in prosenchyma and their 

 firm continuity admirably adapt them. An equally important 



