220 LIFE : OUTLINES OF GENERAL BIOLOGY 



that live in the soil. Similarly our knowledge of soil nematodes is 

 still very fragmentary. Some recent researches by Dr. David 

 Robertson show clearly that there are many different species and 

 that the number of individuals is often huge. 



Taking the second point first, we notice that, as an average of 

 ten samples, Robertson found in a cubic inch of soil 65 nematodes 

 from oat stubble, 70 from two years' pasture land, 105 from one 

 year's grass, 115 from a clayey oat field. Of course that means 

 billions to the acre, for even the 55 nematodes per cubic inch of 

 potato land means two billions per acre six inches deep; and the 

 low census of 35 per cubic inch of clayey turnip land means far 

 over a billion per acre. There can be no doubt that the nematode 

 or threadworm population is enormous. It would be interesting to 

 have records from wild soil of various kinds. We know that there 

 are multitudinous individuals of Tylenchiis hordci (if this species- 

 name still survives), boring into the roots of the sand-binding 

 grass (Eljonus) at the exposed shore immediately to the north of 

 Aberdeen. This nematode makes galls on the thread-like roots; 

 and the larvie can survive, as we have verified, for a couple of 

 years the drjTiess of a shelf above steam heating pipes. After two 

 years of this drought they moved about soon after the galls, soaked 

 in water, were teased out. 



The second point is the diversity of species, for Robertson found 

 eleven different kinds of Dorylaimus, two of Aphelenchus, two of 

 Motionchus, three of Rhahditis, one of Ccphalobus (the viviparous 

 C. filiformis), and two of Tylcnchus. The disease of "Cauliflower" 

 in strawberry plants is due to Aphelenchus fragaricv, and A. olesisius 

 does much damage to many plants. "Tulip-root" in oats is due to 

 Tylcnchus dipsaci and "ear-cockles" in wheat to T. iritici. It should 

 l)e noted that many of the soil nematodes are strictly saprophytic, 

 that is to say, they feed on rotten organic matter. Only a minority 

 arc known as parasites in plants. One species may devour another; 

 thus Steiner and Heinly observed that an individual Mononchus 

 papillatus devoured 83 larva; of Heterodera radicicola in twelve 

 weeks. Ideology is very intricate. 



Returning to soil bacteria, important agricultural developments 

 seem approaching. It is now a comparatively old story that under 

 certain conditions, of intensive culture especially, soil may be 

 greatly improved after heating to a temperature sufficient to kill 

 out undesirable germs, and other plant-enemies as well. But while 

 the farmer has increasingly been applying costly manures of various 

 kinds, and this with the approval and even urge of his scientific 

 advisers — witness the potash-salts of the Alsatian mines, or the 

 nitrates prepared from the nitrogen of the air — some of these experts 

 now Ix^gin increasingly to doubt whether so much of such manuring 

 be really the best course after all. For they are finding reasons to 



