hGoi:2 



to her fine work. These nematodes most often are unable to survive 

 desiccations, although some of the drought resistant species may also 

 occur in wet situations. 



The ability to survive desiccation is not possessed to the same extent 

 by all speciesj in some it is only feebly developed, in others, very 

 strongly so. In some it is restricted to certain developmental stages; 

 in others it occurs in all stages. The most extreme case I know of is 

 Plectus rhizophilus ♦ In the transect, it is number 76, the only species 

 really competent to build up a dense population in habitats where water 

 is rarely present and, when present, likely to disappear again within a 

 few hours. Incidentally, this nematode is better at utilizing the few 

 drops of dew than is the moss with which it lives. 



It would, of course, be nice to know exactly how much water must be 

 present in order to enable nematodes to be active, but the question 

 cannot be answered in terms of absolute amounts. In one particular 

 soil containing about 60^ of organic matter, large nematodes stopped 

 activity at a water content of about 33^. In a sand, practically with- 

 out organic matter, the nematodes' activity ceased at about 2% of water. 

 As perceived by the nematodes, the two percentages represent an identi- 

 cal moisture status, and they determine the limit between available and 

 non-available water. Although we cannot express availability of water 

 in terms of volume or weight, we can — and I would add that we always 

 should — express the water status in terms of water tension or some 

 derived unit of measure. In doing so, it comes out that nematode activ- 

 ity stops a little below the wilting range of plants. Several micro- 

 organisms are able to exploit water that is more firmly bound to soil 

 particles; Dr. Bartholomew sets their limit at about pF 5.6. I know 

 little about this, but it is quite reasonable that they are more effi- 

 cient in extracting water held at high tensions, partly because the 

 organisms are smaller and, thus, able to exploit minute soil crevices 

 and partly because some fvingi and bacteria are reported to live at very 

 high osmotic pressures. 



We know far too little about the details to enable us to be precise 

 about the water requirements of nematodes. However, very promising 

 results have been obtained by Dr. Wallace at Rothamstead in connection 

 with the hatching of cysts. We can say with a fair degree of certainty 

 that nematode activity stops before, perhaps well before, the wilting 

 range of higher plants is reached. Some nematodes will only stop 

 activity at this critical value while others will die. 



It may now be asked if it is all that important to know how different 

 nematode species behave with respect to critical water tension and 

 desiccation. I can only answer that it is. Two examples may illustrate 

 the point: 



(1) We want to determine the density of active, free-living 

 nematodes or free-living stages of a plant parasite. VJe 

 can do that by several methods, but since the techniques 

 involve water treatment, the number we arrive at has 



