6o DISEASES OF CROP-PLANTS 



information suggests possibilities in regard to the existence or 

 establishment of control on lines familiar in the case of insect 

 pests. 



The nematodes parasitic in plants are typically minute, one 

 millimetre (^V inch) in length or less, and unless massed in 

 large numbers are invisible to the naked eye. The head is 

 provided with a pore-like mouth through which, in many species, 

 can be protruded a needle-like organ, the oral spear, with which 

 punctm-es may be made in the plant tissues to liberate the juices 

 or allow of the passage of the worm. The degree of development 

 of the spear is to some extent an index of the mode of Hfe of the 

 species. Behind the mouth parts is a tubular oesophagus with 

 a bulbous expansion, and salivary glands which, like the spear, 

 may be well developed or rudimentary. It is probable that 

 the latter provide in some species an excretion toxic to the tissues 

 attacked, as in some sucking insects. There is next a straight 

 gut terminating in an anal orifice near the tail end of the worm. 

 The method of feeding, in conformity with the structure described, 

 is by the ingestion of fluids only. 



In most of the nematode diseases of plants which have been 

 made known the worms have been endoparasites, existing for 

 the greater part of their active life within the tissues of the in- 

 fested plant. Other species are known to live in the soil and 

 puncture the epidermis of young roots without entering : the 

 effect of which, from the nature of the case, is difficult to estimate. 

 Recently two species, referred to below, have been described 

 which are fully parasitic, but are ectoparasites, ascending the 

 outside of stems and lodging in buds, under leaf-sheaths, or in 

 similar situations, puncturing the tissues and feeding from 

 without. 



The plant-feeding nematodes have a transparent skin, the 

 external layer of which is a highly impenetrable cuticle. The eggs 

 are similarly enclosed. Considering their fragile appearance, 

 both worms and eggs are surprisingly resistant to solutions of 

 chemical poisons. They are somewhat more susceptible to 

 gases, and the best results obtained in the treatment of infested 

 soils by chemical agents are probably those obtained with 

 carbon bisulphide or ammonia. The usefulness of lime in this 

 connection is disputed, but in any case only heavy applications 

 can be expected to have any notable effect. 



The resistance of the worms to dryness varies very consider- 

 ably with the species. It is not uncommon for a period of 

 dormancy in a dried-out condition to form part of the normal 

 life history, and this has been shown to be capable in some species 

 of extending to six months and more with little mortality or 

 loss of vigour. The larvae of the wheat nematode, which assume 

 a resting condition in altered grains of wheat, have been recorded 

 as resuming their activity after being dried for 27 years. Where 

 a condition of dryness does not come in to cause cessation of 



