39P THE POPULAR SCIENCE MONTHLY, 



opment of the parasite. The tissues in which the latter feeds must 

 thus become diseased, primarily through lack of nutrition, and so final- 

 ly develop in an abnormal manner, as is seen to be the case in the 

 often enormous knots which accompany the growth of the mistletoe 

 upon the oak. Such excrescences often reach a diameter of three or 

 more feet. A secondary feature of such diseases is then developed in 

 the readiness with which such hypertrophies often yield to decay, or 

 in the decay which is introduced into the various tissues of the host 

 wherever the parasite penetrates. It is evident that diseases of this 

 character may be, and usually are, of a strictly local nature, and, in 

 the early stages at least, it is easy to remove both the disease and 

 the cause by amputation. When local action has been long continued, 

 however, the highly morbid condition of a limited portion of tissue 

 may in time find sympathy in adjoining parts, and so by degrees the 

 whole system become involved in a chronic disorder. We may thus 

 remove the cause, but additional treatment will be essential to re- 

 store the system to its normal condition. 



In the second class of causes we have the cryptogamic parasites, or, 

 more properly, the saprophytes, to contend with. These plants, like 

 the parasites proper, are incapable of providing their own nourishment 

 from the soil and air, and so must depend for their growth upon al- 

 ready-formed organic matter. But this is not all : it is characteristic 

 of their growth that they live upon organic matter which is in an ac- 

 tive state of decomposition, and it will thus be easy to see that they 

 are not far removed from being the cause of the decomposition in bodies 

 which have already ceased to live. In their action upon non-vitalized 

 matter, it is quite possible that they are the active promoters of dis- 

 organization ; but the case is somewhat different with the living or- 

 ganism. Here the growth of the saprophyte has to contend with the 

 vitality of the host, and, so long as this latter is normally maintained, 

 it is most probable that the intruder will fail to gain sufficient hold to 

 exert any appreciable injury. But the struggle continues, and if, by 

 reason of accident or peculiar conditions of environment, the vitality 

 of the host be reduced below certain limits, then the saprophyte or 

 parasite, as the case may be, at once exerts a preponderating influence 

 which is highly deleterious. Or, again, if the plant be diseased through 

 the operation of other causes, then the fungus can exert its influence 

 to produce secondary features of an already disordered condition. 

 These views find confirmation in the general action of fungi upon tis- 

 sues. It is observed that they are more or less abundant in the rough 

 outer bark and on the surface of most plants ; but, though they are 

 present, their growth is limited, and confined to those tissues which are 

 either dead or of very low vitality, while the plant suffers in no wise 

 from their presence. Let the plant be injured or diseased, however, 

 and at once the parasite gains a firmer hold, the tenacity of which will 

 increase continually until remedial measures are applied. Thus, we 



