34 REPRODUCTION 



mold should light upon it and germinate and get a footing, 

 we have two distinct problems. One of them, the 

 scattering of the young of the species far and wide, was 

 mentioned in the preceding section. The other grows 

 out of the fact that this fish, before it has completely 

 decayed, can- support many plants of the fish mold. 

 Therefore it is a clear gain to the species if this mold 

 has a way whereby it can quickly take possession of the 

 fish. The same problem we find in the molds on a culture 

 made of a slice of bread in the laboratory. Both of these 

 classes of organisms have devices for taking full 

 possession of a limited area quickly, as well as devices 

 to extend the species to other spots. In the fish mold, 

 aside from the branching threads of the original parents 

 which spread rapidly over the fish, there are small, short- 

 lived swimming spores that germinate quickly and start 

 new plants near at hand. They are free swimming and 

 might move to considerable distances in the water, but 

 they are attracted by the chemical changes in the water 

 due to the decaying flesh, and so very many of them do 

 not get away from this attractive spot. Thus a very 

 luxuriant growth occurs in a very brief time on the fish. 

 This fact produces the disease and death of many fish, 

 but it means that the mold is being successful. 



3. The Effect of the Medium on the Method of Scattering. 

 Plants producing spores grow in air, in water, and in 

 solids, as soil, etc. In the last case, however, it usually 

 happens that the plant sends the spore-bearing branches 

 to the surface, so that the spores are set free either in 

 the air or in water. The problem of scattering the spores 

 must be solved differently in the air and in the water. 

 The water is more heavy and supports the spores better. 

 Furthermore, it keeps them moist, and spores prepared to 

 live in water need not have such thick walls. They may 

 indeed lead a very active life, swimming here and there 

 quite independently of the currents in the water. This is 

 true of many spores of algae and of a few fungi. If they 



