Minnesota Plant Life. 487 



an illustration. Suppose some observer stationed in an air- 

 ship many miles above the battle-field of Waterloo. Per- 

 haps by the use of powerful glasses he could have made out a 

 vague commotion far below him, but he could scarcely have 

 distinguished what actually was happening. To an observer 

 on the hills near by, the battle would have presented a very 

 different meaning. Thus, the earlier students of living sub- 

 stance, because they could not see all that is revealed to the 

 observer of to-day, fell into the error of thinking that they had 

 under their microscopes a "structureless slime." 



Living substance is by no means homogeneous. Sometimes 

 it shows a fibrous structure made up of innumerable meshes 

 and threads. Again it appears as a foam or emulsion consisting 

 of an intricate combination of larger and smaller bubbles. At 

 one time, when more saturated with water, it is liquid, plastic 

 and mobile. At another, when like a sponge it has expelled a 

 portion of its moisture, it seems almost solid and is passive 

 and immobile. Thus the living substance in the active cells of 

 a growing hair and in the dormant cells of an ungerminated 

 seed may seem very different. By the absorption or expul- 

 sion of water living substances may pass from one condition to 

 the other. 



Chemical composition of living substance. The following 

 chemical elements are components of living substance : carbon, 

 oxygen, hydrogen, nitrogen, sulphur and phosphorus. In the 

 higher plants the following metals are also necessary for the 

 formation of living substance, although they do not become 

 essential components of protoplasm : iron, potassium, mag- 

 nesium and calcium. Numerous other elemental substances, 

 such as sodium, lithium, manganese, silicon, chlorine, bromine, 

 and iodine are frequently found in the bodies of plants; but 

 they do not appear to be everywhere either essential compon- 

 ents of the living substance or necessary reagents for its con- 

 struction. 



It must not be supposed that all living substance is of exactly 

 the same chemical composition. To say that no two particles 

 are precisely alike would probably be nearer the truth. Al- 

 though the number of the essential elementary components is 

 so small, many atoms of each kind are present and the varia- 



