THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 189 



the simplest, though not necessarily the most correct view, is to consider 

 the Archimycetes as embracing three orders, all of which must be regarded 

 as side lines in evolution, while placing the only order which appears to have 

 contributed to the higher Fungi, namely the Chytridiales, in the Phycomycetes. 

 According to this scheme we are left with three orders : — 



1. Myxomycetales, sometimes called the Mycetozoa, which are 



saprophytes. 



2. Acrasiales, in which the plasmodia readily fragment into separate 



amoeboid cells and which live as saprophytes on dung. 



3. Plasmodiophorales, which are parasites of higher plants. 



We shall refer briefly to the general characters of the Myxomycetales and 

 consider one type of the Plasmodiophorales. 



Myxomycetales 



With the exception of one genus, Ceratiomyxa, to which we shall refer 

 later, all the members of this order live on the surface of dead wood, leaves 

 and the like. 



The diploid organism formed as a result of the fusion of swarm cells is 

 termed a myxamoeba and resembles a true Amoeba. It can migrate by 

 means of pseudopodia, that is, by extending a part of its protoplast followed 

 by a flowing movement of the whole body in the direction of the pseudopodium 

 thus formed. In this way it can move at a not inconsiderable pace. The 

 myxamoeba is devoid of any cell wall, and the protoplast only possesses a 

 rather thicker and tougher ectoplasm through which food material in 

 solution is absorbed. The protoplast can, however, engulf, by flowing 

 around them, small objects such as bacterial cells, which it appears able to 

 digest. In this way the body grows and increases in size, becoming a multi- 

 nucleate structure termed the plasmodium (Fig. 177). Several myxamoebae 

 may contribute to the formation of a plasmodium and any unconjugated 

 swarm cells may also be incorporated into the plasmodium, which may thus, 

 in time, both by feeding and accretion, become a body of considerable size ; 

 in fact, Plasmodia containing as much as a litre of living protoplasm have 

 been found quite frequently. During this period the plasmodium may 

 become coloured, and, though white plasmodia are common, sulphur yellow 

 or pink are characteristic of some species. The plasmodium now contains 

 a very large number of nuclei produced by the division of pre-existing ones. 

 When the plasmodium is mature, sporangium formation begins (Fig. 

 178). This may result in the conversion of the whole plasmodium into a 

 single giant sporangium termed an aethalium, or a number of separate 

 sporangia may be formed. In the latter case the sporangia may be sessile 

 but are more usually stalked (Fig. 179). 



During the differentiation of the sporangium an elaborate system of 

 internal threads is laid down as a result of the filling up of cleavage furrows 

 in the protoplasm of the young sporangium with chitinous material. This 



