2*t SU'iH-iliseaxc (•(Dined l>ii Ihi A't/icunii 



proci'ss, but it, was loiind quite ]>r;ictic;ibli', :iiu| coiiiparalively fow of tiie 

 worms were damaged or broken during these operations. Each seedling 

 was placed in a shallow glass capsule iu distilled water, and bit by bit 

 tlisspcted so as to free tlie tissues from the contained parasites wliich were 

 then left in the water whilst the vegetabh; matter was gradually removed. 



After the dissection the nematodes were collected, concentrated and 

 trnnsferred to a microscope slide oi- slides as the case might require, and 

 then e.xaniiued under the microscope, i^'or the purposes of collection 

 1 first of all used glass capillary pipettes, bub later on abandoned these 

 and used the centrifuge which proved very servicealile. The resulting 

 drop for examination thus contained adults of both se.xes, larvae in 

 many stages of development, and numerous eggs. In making my exami- 

 nations I recorded separately the numbers of males, females and larvae : 

 eggs were not counted. A record was also kept of whatever other kinds of 

 nematodes might be present with the Ti/lenchus. These, however, do not 

 concern us in the present paper. 



In the case of those varieties which gave a large number of deformed 

 seedhngs, the ten most deformed were, as stated above, selected for 

 dissection, whilst iu the other cases as for example, lucerne, sainfoin, 

 white clover, where only a few seedhngs were deformed, the \^liole of 

 them were dissected. 



In the case of lucerne only four seedhngs were defornieil. hut none 

 of these revealed any adult Tyleiichvx on dissection. 



Since the same number of seedlings was not dissected tiiroughoul, 

 it became necessary to decide arbitrarily on some number to take as an 

 average in arriving at the index of susceptibility, and for this purpose 

 the number of deformed lucerne seedlings was chosen, i.e. four. 



Since also index of susceptibihty was being interpreted as equivalent 

 to intensity of susceptibility, the four highest totals of males plus fcnuiles 

 were taken and an average made of these, the resultant figure being 

 called the index of susceptibility. Whether the males and females 

 counted are the progeny of a single fertilised female, thus giving a 

 reproduction figure, or whether they are the result of an invasion of the 

 tissues of the host by numerous larvae which have attained sexual 

 differentiation and maturity witliin the seedling, the data available are 

 not sufficient for one to say, since we do not know enough of the life- 

 history of the parasite to say definitely which is the infective larval stage. 



The figure arrived at in each case gives us, I think it will be agreed, 

 an expression of the suitability of the host plant for the needs of the 

 parasite, and this is what the investigation was designed to reveal. 



