Minnesota Plant Life. 489 



tain specialized portions, is a form of "light-engine" by which 

 the energy of the sunlight is set at work building up starch 

 out of such simpler substances as carbonic-acid gas and water. 



The food of plants. In general the diet upon which a plant 

 can live is simpler than that necessary for an animal. The 

 important elements are principally obtained from sources as 

 follows : Carbon is collected from the carbonic-acid gas of the 

 atmosphere ; oxygen both from the atmosphere and from water ; 

 nitrogen from salts in the soil, but not from the atmosphere; 

 and hydrogen from water. Sulphur, phosphorus and the 

 metals are obtained from soluble salts in the soil. In excep- 

 tional instances the sources may be other compounds than those 

 mentioned. Thus nitrogen and possibly hydrogen may be 

 taken from ammonia products, but this would be very unusual. 

 What has been stated refers to green plants alone, for plants 

 without leaf-green demand a much more complex diet. Their 

 carbon they may obtain from sugars and from proteids ; their 

 nitrogen from proteids ; their oxygen and hydrogen from water, 

 from sugars or from proteids ; their sulphur and phosphorus 

 from the soil, or perhaps from proteids ; and their metallic sub- 

 stances from soluble salts in their substratum. 



The plant does not ingest solid particles of food as does the 

 animal — though to this certain carnivorous plants are quasi ex- 

 ceptions — but receives its nutriment either as gases, liquids or 

 solutions. The absorptive tract of the plant is often specialized 

 so that one area is fitted better for absorption of gases and 

 another for absorption of liquid solutions. Thus in the ordi- 

 nary green plant it is the foliage that characteristically absorbs 

 carbonic-acid gas and oxygen, while the root-system absorbs 

 water with nitrogen compounds and metallic salts in solution. 

 Aquatic plants are more generalized, as are also air-plants — 

 although the latter use their exposed root-system very much 

 as if established on a firm soil. Humus plants and parasites 

 obtain their food from the bodies of other plants or animals 

 or from decaying organic substances. 



Partnerships for obtaining food. Some extraordinary part- 

 nership arrangements for securing food have been developed 

 in the plant world. Thus, in the lichen-body a fungus and an 

 alga live in partnership and obtain their food quite as if they 



