50 The Mycetozoa, and 



extensive exposure to the action of the sun and of the 

 atmosphere if they operated equally all round. We say 

 the most natural, as it would result from an equal and 

 universal outward growth, but for the purpose of exposing 

 its surface, the globe must be mounted on a stand ; but as 

 the lower part will be of less value than the top and sides, 

 because less exposed to the action of the sun, it will be 

 convenient that the globe form shall be modified : and this 

 has been sometimes attained by horizontal, sometimes by 

 vertical expansion. Some such physical necessities seem 

 to have influenced the shape of trees ; and similar ends 

 are, we suppose, subserved by the dendroid forms of the 

 capillitium in Comatricha and Stemonitis. How has the 

 chasm between the need and the supply been filled up in 

 these minute organisms or in the stately oak ? 



Another fact which creates further varieties in the form 

 the sporangia is the presence of lime in the capillitium 

 and in the coats of the sporangium. In this presence of 

 the carbonate of calcium in the sporangium, a character 

 has been found for one of the subdivisions of the myxies, 

 the so-called Calcarinece. In some cases the lime is found 

 in small grains in the substance of the covering membrane, 

 in other cases it is found in star-shaped crystals lying on 

 the outside of the membrane. These are very beautiful 

 objects, and may both be seen in the family Physaracece. 



In some cases the walls of the sporangium alone have 

 the lime and the capillitium is without it ; in many other 

 cases the lime is found also in the capillitium, and that in 

 different forms. We have already in our sketch of the life- 



