344 



BOTANY 



PART II 



Each liaploid nucleus tlius formed becomes the nucleus of a spore. In Ceratiomyxa 

 on the other hand a large proportion of the nuclei degenerate after the reduction 

 division, and the spores may include a degenerating nucleus as well as tlie normal 

 one. From the latter by two successive divisions four nuclei are formed and are 

 pi-esent in the ripe spore. Another division occurs on germination, so that 

 ultimately eight swarm-spores are produced from each spore. 



In the structure of their swarm-spores and myxamoebae the Myxomycetes show 

 their derivation from organisms of the nature of the Flagellata. 



Very large plasmodia, often over a foot in breadth, of a bright yellow colour 

 and creamy consistency, are formed by Fuligo variajis {Aethalium septicum), and 

 as the " flowers of tan " are often found in summer on moist tan bark. If exposed 



to desiccation, the plas- 

 modia of this Myxomy- 

 cete pass into a resting 

 state, and become con- 

 verted into spherical or 

 strand - like .sclerotia, 

 from which a Plasmo- 

 dium is again produced 

 on a further supply of 



Fir;. 2JT. — Leocarpus fragilis. 

 Groups of sporangia ui)on a 

 Moss. (Nat. size.) 



water. Finally, the whole 

 Plasmodium becomes transformed into a dry cushion 

 or cake-shaped fructification of a white, yellowish, or 

 brown colour. The fructification, in this instance, is 

 enveloped by an outer calcareous crust or rind, and is 

 subdivided by numerous internal septa. It encloses 

 numerous dark violet-coloured spores, and is traversed 

 by a filamentous capillitium, in which are dispersed 

 irregularly-shaped vesicles containing granules of cal- 

 cium carbonate. A fructification of this nature, or 

 so-called aethalium, consists, therefore, of a number of 

 sporangia combined together, while in most of the 

 Myxomycetes the sporangia are simjde and formed 

 singly. 



The structure and nature of the sporangia afford 

 the most convenient means of distinguishing the dif- 

 ferent genera. The usually brown or yellow sporangia 

 are spherical, oval, or cylindrical, stalked (Figs. 255, 

 257) or not stalked (Fig. 256). They usually open by 

 the rupture of the upper portion of the sporangium 

 walls, the lower portion persisting as a cup (Figs. 255, 

 £, 256, A). In Cribruria (Fig. 25."i, C) the upper part 

 of the wall of the sporangium, which contains no 

 capillitium, becomes perforated in a sieve-like manner. 

 In Stcmonitis (Fig. 255, A) the whole peridium falls 

 to pieces, and the capillitium is attached to a columella, which forms a con- 

 tinuation of the stalk. 



PlasmocUopIiora Brassicac ('•'*), one of the few parasitic Slyxomycetes, causes 

 tuberous swellings on the lateral roots of various sj)ecies of Brassica. Its 

 multinucleate myxamoebae occur in numbers in the cells of tlie hypertrophied 

 I)arenchyma of these swellings ; after tlie contents of the host-cell have been 

 exhausted they fuse into plasmodia, and tliese, after repeated nuclear divisions. 



Fio. 250. — Trichia varia. A, 

 Closed ami open sporangia 

 (x G); ]l,ii fibre of tlie ca- 

 pillitium (x 240); C, spores 

 (X 240). 



