AND FACTORS WHICH INFLUENCE THEM. 145 



lime. Both sessile and stalked sporangia are frequently produced 

 by one plasmodium, and in such cases the presence or absence 

 of a stalk evidently depends upon contingent rather than specific 

 conditions. 



The morphological effect of the presence of lime in sporangia 

 is well indicated by the large proportion of species to genera in 

 the Calcarineae as compared with other genera and species. The 

 figures are : 



Genera percent. Species percent. 



Calcarineae 16 0'33 129 0*52 



Other Endosporeae . . . 32 0-67 116 0*48 



48 1-00 245 1-00 



These figures show that more than half of the species belong 

 to the Calcarineae, although the genera containing them are 

 but one-third of the genera included in the group. Under the 

 varied conditions of environment in different parts of the world, 

 lime in sporangia tends to multiply the number of characters 

 regarded as specific. This is strikingly illustrated by the genus 

 Physarum, which is divided into fifty-seven species, whereas the 

 average number of species in a genus of Mycetozoa is five. From 

 the fact that only a portion of the group contains lime, and that 

 sporangia without lime are occasionally found even in the Cal- 

 carineae, we may conclude that calcium carbonate, although 

 utilised by the plasm in which it occurs, is not an indispensable 

 element of it. When afterwards eliminated it is deposited in 

 a variety of ways, sometimes as a central ball within the 

 sporangium, frequently in the form of a columella, generally in 

 the stalks or embedded in the sporangium walls, or forming an 

 outer crust of stellate crystals. There are two singular cases 

 which also present a contrast. In Physarella there are cal- 

 careous spines attached to the sporangium walls and pointing 

 inwards until the walls are ruptured. In Physarina the walls 

 are covered with calcareous protuberances pointing outwards, 

 resembling the spines of an echinus. It is appropriately named 

 P. echinocephala. 



In all sporangia of Mycetozoa, probably without exception, 



