7 6 



PHYSIOLOGY. 



sion, thus drawing water forcibly through the protoplasmic mem- 

 brane. Since it does not filter out readily, the increase in 



Fig. 65. 

 Spores of mucor, and different stages of germination. 



quantity of the water in the cell produces a pressure from within 

 which stretches the membrane, and the elastic cell wall yields. 

 Thus the gonidium becomes larger. 



180. How the gonidia germinate. We should find at this 

 time many of the gonidia extended on one side into a tube-like 

 process the length of which varies according to time and tempera- 

 ture. The short process thus begun continues to elongate. This 

 elongation of the plant is growth, or, more properly speaking, one 

 of the phenomena of growth. 



181. The germ tube branches and forms the mycelium. 

 In the course of a day or so branches from the tube will appear. 

 This branched form of the threads of the fungus is, as we will 

 remember, the mycelium. We can still see the point where 

 growth started from the gonidium. Perhaps by this time several 

 tubes have grown from a single one. The threads of the myce- 

 lium near the gonidium, that is, the older portions of them, have 

 increased in diameter as they have elongated, though this increase 

 in diameter is by no means so great as the increase in length. 

 After increasing to a certain extent in diameter, growth in this 

 direction ceases, while apical growth is practically unlimited, 

 being limited only by the supply of nutriment. 



182 Growth in length takes place only at the end of the 

 thread. If there were any branches on the mycelium when the 



