CH. VIII. MORPHOLOGY AND COURSE OF DE VELOPMENT. MYXOMYCETES. 435 



as in Trichia and Arcyria, or is separated from it by a transverse wall ; if the septum 

 is convex upwards it is termed the columella or central column (Fig. 189). The cavity 

 of the stalk varies in breadth in different species and contains nothing but air, as in 

 Physarum hyalinum, or is filled in the manner which will be described below. 



The structure of the membrane of the sporangium in most of the species which 

 contain no calcium, Licea, Perichaena, Cribraria, Arcyria, Trichia, &c., is usually such 

 as has now been described. In some of these it also contains coloured granules of an 

 organic substance, the nature and origin of which have yet to be ascertained ; these 

 granules are imbedded in the stronger ridges of thickening matter in Cribraria and 

 Dictydium ; in Perichaena liceoides they lie singly or in groups on the outer surface. 

 The olive-brown outer layer of Licea flexuosa shows an irregularly granular structure 

 throughout the entire thickness. 



FIG. 190. Physarum leiicophaetim, Fr. a spor- 

 angium seen from without. * sporangium divided 

 in half and the frame-work of the capillitium 

 exposed by removal of the spores. Magn. 25 times. 



FIG. 191. Physarum letuophaeum, Fr. Piece of the wall of a 

 sporangium with tubes of the capillitium attached and spread out in 

 water ; a points of attachment of the tubes of the capillitium ; * calcium 

 carbonate-vesicles ; to the right on the margin a calcium carbonate- 

 vesicle on the wall. The rest of the wall is covered with calcium 

 carbonate-granules singly or in groups. Magn. 300 times. 



On the other hand the wall of the sporangium in most of the Physareae and their 

 allies (Calcareae of Rostafinski) is incrusted, wholly or in part according to the genus 

 and species, with the calcium carbonate eliminated in the separation of the spore- 

 plasm. In a number of genera, of which Physarum may be taken as a representative 

 (Figs. 190, 191), the calcium carbonate appears in the form of small round granules 

 which either lie isolated and more or less deeply sunk in the wall, or form dense 

 irregular accumulations on its inner surface. In many species the granules of calcium 

 carbonate, especially those that are collected together in heaps, are surrounded by the 

 extruded colouring matter mentioned above, which is yellow in Physarum aureum, 

 P., Ph. sulphureum, A.S. and other species, or more rarely reddish-yellow as in Ph. 



F f 2 



