400 HETERODERA RADICICOLA. 



potatoes proved to be the most convenient material for studying 

 the life-history of the nematode, since they can be easily scraped 

 or cut and harbour the worm for a considerable time in all 

 stages of its development. To a great extent the present obser- 

 vations were based on material consisting of infected potatoes 

 from parts of Natal, the Transvaal and Basutoland. 



Seedlings of tomatoes, potatoes, peas and beans have been 

 used for most of the experiments with the nematode. 



A large number of cultivated and wild plants have been 

 examined, and a list of plants found to be infected in Nature 

 has been made (see page 412). It is probable that such wild plants 

 serve as reservoirs for the nematode, whence it spreads to new 

 cultivated hosts. 



Cultures of the larvae of Heterodera radicicola were made in 

 wet sand and in water, but it was found that the development of 

 the larvae was arrested and that no further development occurred 

 unless the worms were able to enter the roots of a new host 

 plant . 



The adult females of Heterodera are usually visible as small, 

 pearly white, glistening bodies, about the size of a pin's head. 

 They are chiefly situated in small cavities in the peripheral 

 portion of the infested root, and are most conveniently removed 

 by teasing the surrounding root tissues under water and lifting 

 the worms with a pipette. For isolating the smaller intermediate 

 stages of the worm the use of a binocular microscope is advan- 

 vageous. 



Owing to the transparent nature of the cuticle it was found 

 possible to determine most of the detailed anatomy of the 

 Heterodera without staining. Much time was given to the study 

 of living material. Permanent preparations were not very satis- 

 factory owing to the impermeability of the cuticle to most fixatives 

 and stains. The fixatives found most useful were hot 70 per cent, 

 alcohol, Carnoy's fluid and Bouin-Duboscq fixative. For staining 

 it was found that a fairly weak solution of acetic-acid-carmine 

 acting overnight gave better results than haematoxylin, methylene 

 blue and other stains. For mounting specimens permanently, a 

 five per cent, solution of carbolic acid in alcohol was found very 

 convenient. This mounting medium also helped to clear specimens 

 and could be easily ringed with enamel. 



It may be mentioned that the size of root galls cannot always 

 be regarded as a reliable indication of the extent of infestation 

 by the parasite, as this may vary with different hosts. 



The Life-History of Heterodera radicicola. 



The life-history of Heterodera radicicola as found in South 

 African plants may now be considered in some detail. 



The Eggs (Plate VI, Figs. 1-11).— The eggs are mostly easily 

 obtained in numbers by opening the gravid female worm. They 

 are usually found still within the oviduct in the unsegmented 

 condition, but occasionally the segmentation is quite far advanced. 



