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