26 



acted on by diastase, and thus rendered soluble, that it can be taken up by 

 ■the sap and used as food in the building up of new tissues. The failure to 

 secrete sufficient diastase thus causes a condition quite comparable to that 

 of severe indigestion in man or the higher animals. 



By far the greater number of plant diseases are caused by the action of 

 parasites. The number of kinds of parasites that infest plants is very 

 great. Probably no plant of economic importance is free from thera, and 

 the more widely cultivated crops have to contend with a formidable num- 

 ber of parasitic foes. These may be either animal or vegetable, and they 

 belong to widely difiering groups. In the vegetable kingdom plant para- 

 sites are found among the slime moulds, the bacteria, the green algae, the 

 fungi, and a few even among the flowering plants. In the animal kingdom 

 they are less widely scattered, being found only among the nematode worms 

 the mites and the insects. It is in the great group of chlorophylless plants 

 called fungi that we find by far the greatest number of plant parasites. 

 The diseases known as smuts, rusts, mildews, leaf-spots and moulds are all 

 caused by fungus parasites, while many of the blights, rots, and wilts are 

 also due to them. 



All parts of the plant are liable to be invaded bv parasites. Roots, 

 stems, leaves, flowers and fruits each have their special enemies. The sur- 

 face only may be the point ofattftck, or the parasite may burrow deeply 

 in the tissues. The nature of the injury caused will depend on the habit, 

 and structure of the host plant, on the point of attack, and on the character 

 of the parasite. In some cases it may be little more than the loss of a cer- 

 tain amount of food material,, the host and the parasite bting so adjusted 

 to each other that the latter liv^es with a minimum of inconvenience to the 

 former. Plants of wheat or oats infested by smut show very little incon- 

 venience from the presence of the mycelium of the former in their tissues. 

 It is only at maturity when, instead of ripened grain, we find the black 

 powdered masses of fungus spores that the extent of the injury is sus- 

 pected. Such cases, however, are rare. There are usually secondary com- 

 plications that do for more harm than the mere loss of food. Thus the coat- 

 ing of the surface of leaves by external growths of mildews and sooty moulds 

 shuts ofi the light from the chlorophyl bodies, partially preventing pho- 

 tosynthesis, the process of starch formation. The presence of internal 

 parasites often excites a morbid growth of the plant tissues causing galls, 

 knots or other deformities, or they may cause an excessive formation of 

 gums or resins. In other cases the parasites may multiply so greatly in 

 the tissues as to plug the ducts in the vascular bundles, shutting off the 

 ascending sap and thus causing the sudden wilting and death of the entire 

 top. The rotting of the roots may cause a similar wilting. A bacterial 

 parasite causes the fermenting of the sap in the soft cambium layer of pear 

 and apjjle trees causing the sudden death of considerable branches. 



Different fungi have acquired the power of parasitism in different de- 

 grees. The true parasites like the rusts and smuts have the power of tak- 

 ing their nourishment directly from the living protoplasm of their hosts. 

 In most cases they do not kill the tissues in which they are embedded 

 though they may interfere seriously with their normal functions. Other 

 fungi that normally live on decaying vegetable matter have developed the 

 power under certain conditions of penetrating tissues that are still living. 

 These are called facultative parasites. They are not able as a rule to take 

 nourishment directly from the living protoplasm as do the true parasites, 

 but they push their hyphae into or between the living cells of the host, 

 And by the secretion of poisonous acids and enzymes kill them and render 



