10 THE MYXOMYCETES 



sen ting various groups, it shrinks to a small disk-like base at the 

 foot of each sporangium. Only rarely is it entirely absent. The 

 sporangia may be sessile or stalked, and the stalk, when present, may 

 be solid, and then usually horny or calcareous, or hollow, when it is 

 commonly filled with waste material or spore-like cells. Not infre- 

 quently the stalk is prolonged into the interior of the sporangium as 

 a conical, cylindrical or swollen columella. The columella is particu- 

 larly well developed in Stemonitis, Comatricha, Lamproderma and 

 related genera. 



The essential parts of the sporangium are the enclosing wall and 

 the mass of spores. In addition a capillitium is often present, composed 

 of netted tubes or thread-like processes. The sporangium wall is not 

 cellular in structure but is in the nature of an excretion. In some 

 species it is relatively constant in thickness and appearance; in others 

 it is variable, the differences being presumably due to variations in 

 the external environment during its formation. In most species of 

 Stemonitis and Comatricha it is exceedingly delicate and disappears 

 as soon as the sporangium is mature. In Arcyria the upper portion 

 is broken into flakes by the expanding capillitium and falls away, 

 leaving only the persistent cup-like base. In the crateriums and in 

 many of the badhamias, physarums and didymiums it remains for a 

 long time, while in the didermas it is often well preserved long after 

 the spores have been discharged. 



The capillitium is sometimes completely lacking, as in Licea and 

 Cribraria. In Badhamia it is in the form of a network of limy tubes; 

 in Physarum and related genera the lime is aggregated into nodules 

 which are connected by a network of nearly or quite limeless tubules. 

 In Stemonitis, Comatricha and Lamproderma the capillitium arises as 

 branches of the columella. In Hemitrichia and Arcyria it takes the 

 form of a network of elaborately sculptured tubes, while in Trichia 

 the threads are like those of Hemitrichia, but shorter and separate. 

 In Lycogala, Reticularia and Enteridium a true capillitium is lacking 

 but its place is taken by a pseudocapillitium composed of coarse 

 tubes or frayed or perforated plates. The distinction is based on the 

 method of formation, the true capillitium being formed of materials 

 laid down by intra-protoplasmic secretion on the walls of vacuoles or 

 tubular invaginations, while the pseudocapillitium is the direct product 

 of the degeneration of a portion of the protoplasm itself. Since the 

 development of only a comparatively small number of species has 

 been studied, the mode of formation of the capillitium must in most 

 instances be inferred, but the differences between the two structures 

 are such as to lead to little uncertainty in practice. 



