42 Letters on the Diseases of Plants. 



chemicals, the method has been in all cases drastic, the attempt being to kill 

 all the worms at one fell swoop. Possibly a homoeopathic treatment would be 

 more fatal. It is easy to believe that many of the experiments which have 

 been tried were in reality effective so far as they went, although pronounced 

 unsuccessful because the disease reappeared. My experiments have already 

 shown that the period occupied by the development of the egg of A.us- 

 tralian gall-worm may possibly extend over two months or more. While yet 

 in the egg, the young worm is protected by the shell ; and this protection is 

 a good one. The shells of nematode eggs (as well as the skin of the larva 

 when it is being cast) are comparatively impenetrable. Poisons which would 

 at other stages of life be fatal, can therefore be withstood by embryos and 

 moulting larvae. I may support these statements, which are based on my own 

 observations and experiments,* by the remark that the eggs develop in the very 

 midst of decaying matter. The roots attacked by the disease die and decay, 

 thus giving rise to chemicals of considerable strength and activity. Tet the 

 eggs develop unharmed — quite likely, on account of the impenetrability of 

 their shells. Now, suppose in some of the numerous experiments that have 

 been made, all or most of the larvae actually in the soil and unprotected were 

 killed by thie poison used. It is plain that the remedy was a good one, thus far ; 

 yet, if the eggs and moulting larvae escaped destruction, because protected 

 by their coverings, they would give rise to galls in the course of a few weeks 

 or months, and the experiment would be pronounced a failure. I therefore 

 repeat my suggestion that perhaps a more gradual and longer-continued 

 treatment would be successful with some of the chemicals already tried and 

 pronounced ineffective. Among those tried are kerosene emulsion ; various 

 solutions of arsenic ; bisulphide of carbon ; carbolic acid ; the sulphates of 

 ammonium, potash and iron ; the sulphite, sulphide, and muriate of potash ; 

 hyposulphite of soda ; tobacco dust. 



4. Frost.— Idv. Neal says that if in places where the soil is frozen to some 

 little depth each year the ground be ploughed at times during the cold season, 

 it is reasonable to suppose that great destruction of the gall-worms will 

 ensue. I know of no experiments demonstrating that the worms will not 

 revive from the effects of low temperature, as they certainly vnW from those 

 of long continued dryness. 



5. Brainaqe. — It has long been known that drainage has an important 

 bearing on the spread of T. devastatrix. Currents of water on or beneath the 

 surface of the soil will pick up and transport small and light objects. Those 

 objects of least specific gravity are most subject to the transporting power of 

 water. Of all the constituents of the soil, none probably are more likely to 

 be thus moved from place to place than minute organisms such as the eggs 

 and larvae of gall-worms ; hence the great importance of drainage in connec- 

 tion with root-gall, as well as with the ravages of T. devastatrix. By a good 

 system of surface drainage, surface-water may be so controlled as to spread the 

 disease as little as possible. It is needless to go into particulars, as any farmer 

 can easily devise a system of drains suited to his individual case. This 

 matter must not, however, be overlooked by anyone whose land is infested. 

 It is certain that water is one of the chief agents in the spread of root-gall. 



6. Famine. — Famine is as destructive to gall-worms as to other animals, and 

 there is not the slightest doubt that land kept quite clear of vegetation will, 

 in time, become disinfected — the worms dying of starvation. How long a 

 time would be required is unknown ; probably more than a year. It is not 



* For instance, I have observed that species whicli under ordinary circumstances are 

 instantly killed by osmic acid may withstand the acid for an horn- when moulting. 



