BACTERIAL DISEASES OF THE ANIMAL KINGDOM 127 



coccus is about 0-9 /^ in diameter and grows in clusters. Another 

 is Staphylococcus ' pyogenes albus, which exhibits a white growth in 

 all nutrient media. Some of these Staphylococci have much vitality, 

 thus, S. pyogenes aureus can stand half an hour's exposure at 80° C. 

 without injury. 



We have dealt with the better known of the pathogenic bacteria 

 that attack man and the lower animals. Extended research will 

 probably show that other diseases, e.g. bronchitis, whooping-cough, and 

 measles, are also due to poisons secreted by pathogenic bacteria that 

 have found an abiding place inside our bodies. 



§ 3. BACTEEIAL DISEASES OF THE VEGETABLE KINGDOM. 



In speaking of pathogenic bacteria, we must not forget that under 

 this term must be included those organisms that ravage the vegetable as 

 well as those that ravage the animal kingdom. The general principles 

 underlying the liability to disease are the same. If a plant be injured, 

 the point of injury is a weak spot in its defences, through which the 

 investing bacteria or moulds, which are always present, are sure to 

 eifect an entrance unless the plant can cover up the weak spot by 

 means of a layer of cork or by some other way. In the case of plant 

 diseases, moulds are nearly always the originators. In some cases, 

 they act in conjunction or in competition with bacteria, and in still 

 fewer cases, bacteria only are found. The vine-disease known as Mai 

 vero is an instance in which only bacteria are responsible. Black 

 spots and streaks appear on the leaves of the vine. At first they were 

 supposed to be due to the tannin, a good deal of which is found 

 in this plant, but now it is known that these spots and streaks 

 are due to bacterial action. Again, there is very little doubt that 

 the cankering of trees is in large part due to bacteria. Cankers 

 are irregular excrescences due to the perennial struggle between plant 

 tissues which are attempting to heal up a wound, and some organism 

 or other, which is striving to keep the lesion open. The wound must 

 be one which extends to the cambium of the tree, for this tissue is 

 the only part inside the matured stem capable of forming new plant 

 cells. Insects are usually responsible for the iniliction of the wound, 

 which is of the nature of a deep puncture in the wood. Very little 

 is known of those plant diseases in which both bacteria and moulds 

 are found together. Cases of symbiosis between these organisms are 

 known to occur when disease follows the infliction of wounds on the 



