96 The Mechanistic Conception of Life 



are chemical processes. The question is, How can these 

 chemical forces be brought into relation with the visible changes 

 which take place in the formation of a new organ ? The answer 

 to this question is to be obtained by a knowledge of the 

 mechanics of growth. It is very remarkable that the mechanics 

 of growth forms almost an empty page in the history of animal 

 morphology and physiology. I can refer here only to the few 

 experiments I have made on this subject; but fortunately the 

 subject has been worked out very carefully in plants, and as my 

 experiments show that the conditions for growth in animals 

 are, to a certain extent at least, the same as the conditions for 

 growth in plants, we have the beginning of a basis for work. 



A brief outline of the manner of growth in plants is as 

 follows : Before the cell grows it forms substances which attract 

 water from the surroundings, or, as the physicist expresses it, 

 it forms substances which determine a higher osmotic pressure 

 within the cell than did the substances from which the}' originate. 

 The walls of the cell, or rather the protoplasmic layer that lines 

 the cell-wall, possesses peculiar osmotic properties, in conse- 

 quence of which it allows molecules of water to pass through 

 freely while remaining resistant to the passing through of the 

 molecules of many salts dissolved in the water. The result is 

 that when substances of higher osmotic pressure are formed 

 inside the cell, water from the outside passes in until the pres- 

 sure within again equals the pressure without. The cell-wall 

 becomes stretched and, according to Traube, new material is 

 precipitated in the enlarged interstices, thus rendering growth 

 permanent. This method of growth is most conspicuous, 

 perhaps, in the germinating seed. The rising temperature in 

 spring produces in the seed substances of higher osmotic pres- 

 sure (with greater attraction for water) than the substances 

 from which they originate. The result is that water enters the 

 seed; by the pressure of the water within the cells their walls 

 are stretched out and the seed grows. The chemical and 



