192 PHYSIOLOGY OF NUTRITION 



Total dry weight 



Simple proteins 



Nuclein and plastin 



Asparagin and glutamin 



Lecithin 



Fats 



Sugars , 



Soluble organic acids 



Cellulose 



Hemicelluloses 



The main facts regarding germination may of course be most readily demon- 

 strated from the study of seeds germinated in darkness. Germination in light 

 is identical with that in darkness except for the additional assimilation of carbon 

 and mineral constituents. 1 



Summary 



1. The Cell as Physiological Unit. — The Hfe activities of a plant are the summed 

 activities of its cells. Cell activities are due to protoplasm, influenced by the sur- 

 roundings. In all but the very simplest forms, the protoplasm of each cell consists 

 of the nucleus and the cytoplasm, and both parts are necessary for the life of the cell. 

 Plastids are special parts of the protoplasm. Chemically, the protoplasm consists 

 largely of water and proteins. Over go per cent, of the weight of active protoplasm 

 is water. Among the non-aqueous substances in protoplasm the proteins predominate, 

 forty per cent, of the dry weight of slime-mould Plasmodium (protoplasm mainly) 

 being proteins. 



2. Proteins. — The proteins are chemically the most complex substances in the 

 plant. They are very plentiful in resting tissue (such as seeds), Jess plentiful in active 

 tissue, and least plentiful in mature tissue that has nearly ceased its activities. Full- 

 grown leaves that are still active photosynthetically contain much protein in their 

 chloroplasts. There are several chemical tests generally used for identifying proteins. 



The proteins of plants are considered as belonging to two groups, the simple 

 proteins (such as the albumin of aleurone in seeds) and the compound proteins (which 

 are essential in the protoplasm itself). The simple proteins contain carbon, hydrogen, 

 oxygen, nitrogen, and often sulphur. They are combinations or complexes of much 

 simpler nitrogenous organic compounds, the amino acids. About seventeen different 

 amino acids enter into the simple proteins of plants (and animals also). The simplest 

 of these acids is glycocoll (alpha-amino-acetic acid, CH 2 NH 2 COOH). As examples of 

 the most complicated amino acids may be mentioned cystin (alpha-diamino-beta- 

 dithio-dilactic acid) and triptophan (beta-indol-alpha-amino-propionic acid). The 

 constituent amino acid molecules are joined into groups called polypeptides, and 

 polypeptide groups are united to form the simple protein molecule. The simple 

 proteins, of which there are a large number, act as foods and are not considered here 



1 For a treatise on seed germination see: Detmer, Wilhelm, Vergleichende Physiologie des Keimungs- 

 processes der Samen. Jena, 1880. 



