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There is another method of dissemination that may be of consid- 

 erable importance. Since the spores are produced in such great num- 

 bers they must become scattered over the vegetation nearly every- 

 where, and herbivorous animals must eat them by the thousands. Cer- 

 tain kinds of mushrooms grow only on dung and it has been proven 

 that in some of these, at least, the spores are not able to germinate 

 until they have passed through the alimentary canal of some animal. 

 The animal, therefore, by eating the spores, not only prepares them 

 for germination but deposits them in a place suitable for their growth. 

 An interesting case of this type of dissemination is found in a little 

 fungus called Pilobolus. Piloholus is too small to be called a mushroom 

 but it is a very interesting plant. It grows only on dung, and when 

 its spores are mature it hurls them with such force that they are thrown 

 clear off from the dung pile and on to the surrounding grass. These 

 spores can never grow on the grass, but, if they are eaten by some 

 herbivorous animal along with the grass, they are much more certain 

 to be deposited in a favorable place for growth than if they were sim- 

 ply blown about by the wind. 



Gravity. — The force of gravity and its effect on plants is practi- 

 cally the same all over the surface of the earth. For this reason it is 

 of more interest from a physiological than from an ecological point of 

 view. But it will be of interest to note here, in passing, the importance 

 of the way in which it affects mushrooms. It is the force of gravity 

 which causes the stems of flowering plants to grow upward and the 

 roots to grow downward. Likewise it causes the umbrella type of 

 mushroom to grow upright and with its cap horizontal. If the mush- 

 room encounters an obstacle as it comes up, or if grows from the side 

 of a tree or stump, the stem always curves in such a way as to bring 

 the cap into a horizontal position. The significance of this fact is that 

 if the cap is not horizontal, so that the gills are vertical, the spores, 

 when they fall, will strike against the sides of the gills, and when 

 they do that they always stick fast and never fall off. So sensitive 

 are the gills themselves to the force of gravity that if a cap is laid on 

 a table with one side raised higher than the other by an object placed 

 under it, the gills will gradually move in such a way as to bring them- 

 selves into as nearly a vertical position as possible. 



Air. — The composition of the atmosphere, that is, the relative 

 amounts of the different gases, and the dust particles in it, is of con- 

 siderable importance to some kinds of plants, but ordinarily it is not 

 of any great importance in connection with mushrooms. 



Heat. — A certain amount of heat is necessary for the growth of 

 any plant. There are very few kinds of mushrooms that ever grow 

 at all during the winter. There are many kinds, however, that are 



