VEGETATIVE GROWTH 159 



ment carbon is taken by the fungus mainly in the 

 form of a soluble carbohydrate such as sugar. 

 Nitrogen, sulphur and the other necessary elements 

 are absorbed in the form of various salts, together 

 with water, although nitrogen is sometimes absorbed in 

 the form of very complex compounds. The mycelium 

 also branches in the air above the substratum, but 

 its aerial growth is necessarily strictly dependent on 

 the extent of the growth, and therefore of the absorb- 

 ing surface, in the substratum. 



Here for the first time among the types we have 

 studied we meet with the fixed and branching habit 

 characteristic of plants. The " food " of Mucor exists 

 all round it in the bread or jam or other organic sub- 

 stances which forms its substratum, and the branch- 

 ing habit of its " root " puts it in touch with as much 

 food as possible. 



Oxygen for respiration is obtained from the air, 

 and the necessity of free oxygen is illustrated by the 

 fact that the mould forms a cushion only on and in 

 the top layer of jam in the jam-pot. It is probable 

 that the mycelium cannot normally penetrate into the 

 deeper layers of jam because of the deficiency of free 

 oxygen away from the air. Various species can, 

 however, like yeast, live anaerobically for a time and 

 can produce alcohol and carbon dioxide by splitting 

 sugar molecules without oxidation. 



So long as the fungus can get food, water and 

 oxygen it continues to grow, i.e. the nuclei divide 

 and the protoplasm increases in amount at the tips 

 of the hyphae. The wall covering the tip is con- 

 tinuously pushed out and new wall substance is con- 

 tinuously added to make good the thinning, just as 

 in the growth in size of the embryonic cell of the 



