4 PLANTS AND MAN 



difference m this respect between the living substance of an oak 

 tree and that of a dog. Under the microscope this protoplasm 

 is a grayish material of a semi-liquid consistency and a granular 

 or foamy appearance, largely composed of water. Suspended in 

 the watery portion are numerous kinds of proteins — compounds 

 of carbon, hydrogen, oxygen and nitrogen plus a few salts. The 

 two most significant facts about protoplasm are that it is a mix- 

 ture of proteins and that these exist in a colloidal condition. 

 Proteins, the most complex of chemical substances known to 

 man, are made up of amino acids which combine in several 

 hundred thousand different ways; the wide range in charac- 

 teristics of living things is thought to be due, in part at least, to 

 the possible combinations of proteins which make up the proto- 

 plasm of different species. Colloids, which are suspensions of 

 minute particles of one substance in another, even when of a 

 non-living nature, exhibit in a simple fashion characteristics 

 such as growth which are usually considered more typical of 

 living things. Common colloids are smoke, milk and gelatine. 

 When a drop of copper salt is placed in a potassium ferrocyanide 

 solution, a colloidal membrane forms between the drop and the 

 solution, growing by the addition of materials from either side. 

 The colloidal nature of protoplasm makes possible fluctuations 

 from a jelly-like to a liquid condition, and important surface 

 tension phenomena which are vital in many protoplasmic 

 activities. 



No one knows for a certainty how or when protoplasm first 

 evolved from the non-living and inorganic materials of the 

 earth's crust. There is little doubt, however, that the first mani- 

 festations of life were extremely simple ones, perhaps naked 

 masses of protoplasmic jelly floating about in prehistoric seas, 

 taking in the necessary elements for protoplasm-building through 

 their colloidal membranes and living in a much more primitive 

 fashion than living organisms do today. 



The Unicellular Organism 



Long before the beginning of the fossil record, protoplasm 

 developed the ability to encase itself in a wall more durable 

 than the protoplasmic membrane, and within this protective 



