210 TJie Scottish Natwalist. 



recently been awakened in the salmon disease. That disease is 

 supposed by some to be produced by a fungus, the Saprolegnia 

 ferax. By others it is supposed that the fungus is consequent on 

 the disease. And hence arises in this and in other cases the 

 need of a clear conception oi parasitism^ which may be defined 

 as the state of a living organism living upon the life of a living 

 organism. In any given case in which a fungus is found locally 

 associated with an organised body in a state of vitality, or in a 

 state of disintegration, the question may be raised, Is the fungus 

 a parasite? Is it a fungus which requires a living host for its 

 growth and development? Is the club-root fungus a parasite? 

 Does the club-root fungus demand a living and healthy plant in 

 the tissues of which to work out the cycle of its life ? 



The true answer to this question, if it can be given, is highly 

 important to agriculture. For, if it shall appear that a weak, or 

 sickly, or decaying plant is required as the proper soil of the 

 club -root fungus, then this fungus is not a parasite, and its 

 presence merely indicates antecedent disease arising from some 

 failure of practical husbandry. Something has been done, or 

 something has been neglected, in consequence of which the 

 turnip plant and other plants fall into a state of disease, and then 

 become the prey of the club-root fungus. 



But, on the other hand, if it shall appear that the club-root 

 fungus is a true parasite — a plant living upon the living substance 

 of another plant — then this fungus assumes a difterent attitude 

 towards agriculture. Conditions calculated to secure, in its 

 absence, a healthy crop of turnips, are of no preventive value. 

 If the fungus is a parasite, the proper nidus which it requires is 

 a living and healthy turnip. That there are numerous fungoid 

 parasites waging perpetual war upon living plants is well known, 

 but is not popularly accepted in its full significance. We cannot 

 suppose from any facts known to us that all the potato crops in 

 the country must be in a state of disease and decay before be- 

 coming the prey of tlie potato fungus. The grasses do not 

 require to be in an unhealthy condition before being attacked 

 by ergot and smut and rust. And there seems little doubt that 

 the Hemileia vastatrix, the fungus which destroys the coffee 

 crops, is also a true parasite, attacking the healthy leaves. 



Now, what is the nature of the evidence that the club-root 

 fungus belongs to the class of true parasitic fungi ? 



The root of the turnip is frequently attacked at a very early 

 stage. Turnip seeds may be sown, and the plants matured, in a 



