CH.VIII.—MORPHOLOGP AND COURSE OF DEVELOPMENT.—MYXOMFCETES. 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 leucophaeum, Fr. aspor- FIG. 19%. Physarum leucophaenm, Fr. Piece of the wall of a 
angium seen from without. 5 sporangium divided sporangium with tubes of the capillitium attached and spread out in 
in half and the frame-work of the capillitium water; @ points of attachment of the tubes of the capillitium; 5 calcium 
exposed by removal ofthe spores, Magn. 25 times. 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. 390 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. 
Ff 2 
