67 



is certainly a very animal-like method of nutrition. The plasmodium oi 

 certain species may reach a very large size, irregularly covering a square 

 foot or more of decomposing vegetable matter. It is of about the consis- 

 tency of white-of-egg — ( hence the name slime-moulds ) and slo\vly m.oves 

 along in a kind of creeping fashion, being attracted by moisLure and repel- 

 led by light. After a certain length of time the instincts of the plasm.odi- 

 um seem to change. Then it emerges from imder fallen leaves or the inter- 

 ior of a rotten log and crawls into the most exposed position available, 

 often climbing up the stems of adjacent plants for som.e distance. The reas- 

 on is that spore-formation is now about to take place and in order that 

 the spores may have a good chance of being scattered it is desirable that 

 they should be set free when air currents can reach them freely. The 

 Plasmodium may now form a large number of distinct fruiting bodies 

 ( sporangia ) often showing great beauty and intricacy of structure, or 

 may simply solidify, as it were, into one firm, mass with the interior 

 more or less distinctly divided into sm.all cham.bers, each corresponding 

 to a sporangium,. In either case each sporangium contains an enorm.ous 

 number of spores usually together with certain thread-like structures, 

 and in course of time the spores are scattered by atmospheric agents 

 and the life history is begun afresh. The time taken to pass from spore 

 to spore formation again may be only a few^ days or it may be many 

 months, depending partly on environment and partly on the species. 



As has already been mentioned slime-moulds live typically am.ongst 

 decaying vegetable matter, being especially fond of inoist rotten logs and 

 inasses of decaying l*"a^'es in woods. Here, no doubt, they play to some 

 extent the same role as many fungi, gradually helping to decom^pose the 

 vegetable matter and bring it once more into a condition to be utilized 

 for food by the higher plants. They are therefore to this extent useful. 

 There are, however, a number of organisms usually classed with them 

 which are parasites of other plants, and two of these are the cause of im- 

 portant diseases of crops in this country. These organisms are markedly 

 different from other sHine-moulds, but we know that the adoption of a 

 parasitic habitl^ generally results in profound changes in the structure of an 

 organism, and it is only to be expected that the life history of an organ- 

 ism passed inside the cells of another* plant shouldl be very different fromi 

 that of such an one as we have described above. In particular the com.- 

 plex fruiting bodies of a typical slime-mould w^ould not be required. 

 At all events these parasitic forms are more closely related to the slime 

 moulds than to any other group, so far at least as our present know- 

 ledge goes, and are most conveniently classified with them. We \vill now 

 consider in detail those forms which are of economic interest in this 

 country. 



PIvASMODIOPHORA KRASSICAE Wor. 



This attacks many species of plants, both wild and cultivated, all 

 however belonging to the family Cruciferae, and produces a disease 

 known by the English names of "Club-root," "Finger and Toe" and (in 

 Britain ) Anbury. The two former of these names indicate the peculiar mal- 



