46 BIOLOGrCAL LECTURES. 



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 osmotic changes are the sources for the energy 

 which is needed to overcome the resistance to growth. 



To see whether I could determine what are the mechanical 

 causes of growth in animals, I began at Naples some experi- 

 ments on Tubularia mesembryanthemum. I chose long stems 

 belonging to the same colony and distributed them in a series 

 of dishes containing sea-water of different concentrations. In 

 some of the dishes the concentration had been raised by adding 

 sodium chloride, and in others it had been lowered by adding 

 distilled water. According to the laws of osmosis the amount 

 of water contained in the cells of these tubularians differed 

 with the concentration of the sea- water, the amount being 

 greatest in the most diluted solution and least in the most 

 concentrated solution. If now in reality the mechanics of 



growth are the same for animals as for plants, it must result 

 that the more diluted the sea-water the more rapid would be 

 the growth in the tubularian stem. Of course, at last, a limit 

 is reached where the water begins to have a poisonous effect. 

 It was found, indeed, that within certain limits of concentra- 

 tion the increase in the length of the stems during the same 

 period was greatest in the most diluted and least in the most 

 concentrated sea-water. It is remarkable that the maximum of 

 growth took place not in sea-water of normal concentration, 

 but in more diluted sea-water, though that of course may not 

 be the case in all animals. The following curve (Fig. 13) will 

 give an idea of the dependence of growth upon the concentra- 

 tion of the sea-water in Tubularia. The values for the amount 



