26 The Mycetozoa, and 



they have to select situations affording enough atmospheric 

 exposure to ripen the spores, and enough moisture to enable 

 the swarm spores to swim and move about, and it is no 

 doubt due to the width of the dispersal of the spores that 

 they find these situations, which are, one would suppose, 

 comparatively few. It is probably from this delicacy of 

 the requisite conditions for success that plasmodia are not 

 unfrequently seen to fail in the struggle of life. They will 

 sometimes reach the surface, and commence the formation 

 of the sporangium walls and spores, and then fog off and 

 decay, without ever reaching maturity or producing sound 

 spores. 



The observations with regard to the influence of heat, 

 drought, light, and darkness, on plasmodia may be correct, 

 but it does not follow from them that the needs of the 

 organism dependent on the stage it has reached, or on 

 other circumstances unknown to us, may not also operate 

 on their motions. We know that the sporangia are pro- 

 duced on the surface, but we hardly know whether the 

 organism seeks the surface when it is time to develop 

 sporangia, or develops sporangia when it reaches the 

 surface. 



NEGATIVE GEOTROPISM. It is not only in the motion of 

 the plasmodium as a whole, but in the motion of its parts 

 when it develops sporangia, that we observe an upward 

 movement. Sometimes, no doubt, the sporangia are 

 developed on the under surface or the side of the wood on 

 which they grow. We are inclined to think that different 

 species prefer different situations for the production of 



