74 DISEASES OF CROPS. 



The spores of tlie club-root fungus generally commence 

 the attack in turnip seedlings ; by entering the rootlets 

 with water in an attenuated form. As growth proceeds, 

 the turnip plants become diseased, the foliage droops, and 

 the roots are stunted in growth. If during the summer 

 a thin transverse section of one of the nodular out-growths 

 is examined under the microscope, a large number of cells 

 will be found filled, or nearly filled, with the sKme or 

 plasma of this fungus (Pig. 31, 1). This slime greatly 

 distends the cells of the turnip, and thereby causes the 

 nodular outgrowths upon the roots, known as " clubbing." 

 There are no hyphse. 



As the fungus increases in size, the nodules also increase. 

 In the autumn the slime or plasma breaks up into number- 

 less small spores (Kg. 31, 2), which are surrounded by 

 cell-walls. The spores rest during the winter in the 

 turnip roots. In the following spring (if circumstances 

 are favourable) the spores germinate, when little jeUy- 

 like masses exude from them (Kg. 31, 3). Each little 

 mass is called a zoospore, and is provided with a cilium (a 

 " vibrating tail "), which enables it to move over moist sur- 

 faces. The zoospores of P. hrassicce are always changing 

 their shapes — hence they are somewhat similar to certain 

 low forms {AmcebcB) belonging to the animal kingdom. 

 The zoospores of the club-root fungus (like certain of the 

 Monera ^) quickly coalesce and form what is known as a 

 Plasmodium — hence the generic name of the fungus. If 

 infested and rotten turnip roots have been previously 

 thrown on manure heaps, and the manure then distributed 

 over the land, the plasmodia are liberated by rain or the 

 moisture of the soil, and are then ready to commence the 



' See Haeckel's Studien iibei- Moneren. 



