32 Chapter II 



accustom the plasmodia of Physarum to solutions of arsenious acid 

 which at first repelled them in a very striking manner. This adapta- 

 tion manifests itself by movements of the plasmodia and by the 

 change from negative chemiotaxis (repulsion) to positive chemiotaxis 

 (attraction). 



It is impossible in the present state of our knowledge to state 

 precisely the modifications that the plasmodia undergo during this 

 process of adaptation. Stahl supposes that they depend "on some 

 special properties of the plasmodia (probably in a greater or less 

 richness in water)"; and that it is a case "not of simple phenomena, 

 easy of explanation, but of extremely complicated phenomena of 

 irritability." 



It is evident that, in this case of acquired immunity, we have 

 not to do with a question of physical or chemical modification of the 

 solutions employed but solely with reactive phenomena on the part 

 of the living plasmodia. 



After a phase of active life, during which the Myxomycetes move, 

 feed, digest and expel waste products as do the lower animals, 

 there comes a stage when they become immobile and transform them- 

 selves into a number of sporangia filled with rounded spores. Before 

 leaving their animal aspect for that of true plants, the plasmodia 

 exhibit entirely new attributes. They reject all nourishment and 

 no longer ingest foreign bodies ; they avoid the moisture which 

 previously attracted them and cease to shrink from the light. 



Having come to maturity, the Myxomycetes declare themselves 

 true plants and lead a passive life until the new generation comes 

 forth. Most plants are restricted to this passive phase of the 

 Myxomycetes. In these latter it persists only for a short period, 

 whereas in almost all plants it is the permanent condition. It 

 is at this stage that these organisms are liable to the attack of 

 parasites against which it is necessary for them to oppose all their 

 means of defence. Our knowledge of these means of defence is as 

 [35] yet, as I have already stated, very imperfect, and the example of 

 Sclerotinia libertiana (or Peziza sclerotiorum) which has been the 

 subject of the researches of de Bary 1 remains up to the present the 

 one that has been most thoroughly studied. 



This Fungus, belonging to the group of the Discomycetes, invades 

 many species of plants and often produces great .ravages amongst 

 the cultivated plants of our fields and gardens, such as colza, hemp 

 1 Botan. Ztg., Leipzig, 1886, SS. 377, 393, 409, 433, 449, 465. 



