DIV. I 



THALLOPHYTA 



385 



The order Plasmodiophoraceae ( 15 ) contains a few parasitic organisms, the 

 chief of which is Plasmodwphora Brassicae, which causes tuberous swellings on 

 the lateral roots of various species of Brassiea. Its myxatnoebae occur in 

 numbers in the cells of the hypertrophied parenchyma of these swellings ; after 

 the contents of the host-cell have been exhausted they fuse into plasmodia, and 

 these, after repeated nuclear divisions, give rise to the numerous spores, which are 

 set free by the disorganisation of the plant. In the plasmodium a reduction 

 division takes place, the resulting nuclei being those of the spores. The spores 

 germinate like those of Chondrioderma, and the myxamoebae penetrate the roots 

 of a young Cabbage-plant. The formation 

 of true sporangia, however, does not take 

 place, and this Slime Fungus represents a 

 more simply organised or, in consequence 

 of its parasitic mode of life, a reduced 

 Myxoiuycete. 



The systematic position of this order is 

 still doubtful since it presents some cy to- 

 logical resemblances to the Chytridiaceae, 

 which are placed with the Phycomycetes. 



The small order of the Acrasieae occupies 

 a lower position among the Myxomycetes. 

 There are no swarm -spores. Amoebae 

 arising from the spores increase in number 

 by division and then become associated 

 together, without fusion, to form an aggre- 

 gate plasmodium. In the simplest cases 

 this changes directly into a mass of spores. 

 In some genera (Didyostelium), however, a 

 stalk is formed from some of the amoebae 

 which remain sterile and are converted into 

 firm cells ; up this stalk other amoebae creep 

 and form the mass of spores. 



With some reservation the Myxobac- 

 teriaceae ( 16 ) may b.^ placed here. Our 

 accurate knowledge of them is in the first 

 instance due to THAXTEI:, who grouped them 

 with the Bacteria, but according to VAHLE 

 they come nearer to the Myxomycetes. 

 They are widely distributed and live sapro- 

 phytically on the dung of animals, and in 

 habit resemble the Myxomycetes. In the vegetative stage they appear as swarms 

 of rod-shaped small cells connected together by the gelatinous substance secreted 

 by the cells, and exhibit slow creeping movements. Ultimately they form fructi- 

 fications that are usually brightly coloured ; these have the form either of definitely 

 limited masses of spores or of cysts which contain within a firm membrane the 

 numerous spores arising by a transformation of the rod-shaped cells. The cysts are 

 unstalked, or are raised singly or in groups on a stalk, formed, like the wall of 

 the cyst, of gelatinous material derived from the rod-shaped cells excluded from 

 spore formation. In germination the swarms of rod-shaped cells emerge from the 

 ruptured cysts (Fig. 315). 



FIG. 315. A, Myxococcus digitatus, bright red 

 fructification occurring on dung (x 120). 

 B, Polyangium primigenium, red fruc- 

 tification on dog's dung (x 40). C, 

 Chondromyces apiculatus, orange fructifi- 

 cation on antelope's dung. D, Young 

 fructification ( x 45). E, Single cyst ger- 

 minating (x 200)- ( A , B after QUEHL ; 

 C-E after THAXTER.) 



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