304 LIFE : OUTLINES OF GENERAL BIOLOGY 



be simply thought of as based on fatty acids in which a hydrogen 

 atom is replaced by the amino-group NHj.Thus, to take the simplest 

 one of these, acetic acid, the formula is: 



CH3.COOH; 



and if one of the three hydrogen atoms in the CH3 group is replaced 

 bv NH^, we get: 



CH,(NH,) . COOH 



which is one of the simplest amino- acids, called glycine. Many of 

 these amino-acids, of which there are a number o-' great groups, 

 have been prepared synthetically. Many are common in plants; 

 and aspartic acid may be mentioned as a common yet fairly 

 complex example. 



Another group of foodstuffs built up by and used by green plants 

 are the carbohydrates, usually C„H2„0„, in which n is 6, 01 a 

 multiple of 6, even up to several hundred. They include sugars, a 

 very readily transportable form of food; starches, the most con- 

 veniently storable of reserves; and celluloses, forming the walls of 

 vegetable cells. Of each of these there are many kinds, and their 

 chemical structure is still uncertam above the level of the simpler 

 sugars. The simplest of all is formaldehyde, HCOH, a transient 

 substance, probably the first to he synthesised in the green leaf, 

 and which probably enters into the construction of many of the more 

 complex carbohydrates. 



The third group of foodstuffs that green plants make, primarily 

 for themselves and incidentally for animals, are the fats and oils 

 (lipoids). They are very common and familiar to us, e.g. in olive, 

 linseed, and palm oils. They are compounds of glycerine and fatty 

 acid, and in digestion they break down into these anew. Allied to 

 true fats are the lecithins, very abundant in plants, and familiar, 

 along with the protein vitellin, in the yolk of eggs. Fats form an 

 essential constituent of protoplasm, and they play .some part in 

 determining what may or may not pass through the cell-wall. 



FUNDAMENTAL SUSTENANCE OF LIFE 



No ignorance of biological science can be more serious than 

 the still prevalent failure to appreciate the life and work of 

 the green leaves, which in their more than astronomic numbers 

 cover all that is habitable by other life throughout the world. To 

 realise this is an intellectual revolution, indeed one far more prac- 

 tically important than that from Ptolemaic to Copernican, New- 

 Etonian to insteinian, or even from creationism to evolution. For the 



