ones. 



356 A TEXTBOOK OF THEORETICAL BOTANY 



Sometimes there is rapid destruction, and an attack seem.s at least to lessen 

 the growth of the plant. For example, it has been found that tomato plants 

 attacked by Bacterium solanaceanim only grow to half the size of healthy 

 In other instances the eflFect of bacterial attack may be to stimu- 

 late the growth of certain parts of the 

 plant, whereby abnormal structures are 

 host produced. The Witches' Brooms 

 produced on Pines are now generally 

 attributed to bacterial attack. In other 

 cases the effect of the Bacteria is to 

 produce swelling or hypertrophy of 

 the affected parts. One of the most im- 

 portant examples of this is the Crown- 

 gall, caused by Bacterium {Pseudomonas) 

 tumefaciens, which develops on the stems 

 of Apple trees and other plants, where 

 large swellings may be formed, causing 

 great damage (Fig. 342). 



Apart from diseases such as we have 

 mentioned, in which serious damage is 

 caused to the host plant, there are others 

 which only produce spots on the foliage 

 and do comparatively little harm. 



Certain Bacteria live customarily 

 in plant tissue wdthout causing any 

 apparent damage, in fact, both the 

 bacterium and the host plant appear to 

 derive certain advantages from the union. We speak of a condition in which 

 two individuals of different groups live together to their mutual advantage as 

 symbiosis, and we shall consider later other examples of such an arrange- 

 ment. The root-nodule Bacteria, which we shall discuss shortly (p. 359), 

 can be considered as an example of symbiosis, but we may mention here 

 the case of Ardisia crenata. It is a greenhouse plant, grown for its orna- 

 mental red berries and thick evergreen foliage. Here the bacterial infection 

 is local and internal. The Bacteria occur most abundantly at the edges of 

 the leaves, where they fill pockets or cavities (Fig. 343). They multiply 

 enough to make the leaf tissues swell and turn yellowish round the cavities, 

 but the tissue is not killed and the leaves do not fall. 



Bacteria also occur in other parts of the Ardisia plant, particularly in the 

 seeds, and it is by this means that the Bacteria enter fresh plants. If seeds 

 are heated to 60"' C. they can be freed from Bacteria and sterile plants raised. 

 These are always crippled and stunted, so that we may conclude that the 

 presence of the Bacteria has become essential to the health of the higher 

 plant. A similar condition is found in certain tropical Rubiaceae belonging 

 to the genera Psychotria and Pavetta, in which Nitrogen fixation by the 

 Bacteria has been proved (Fig. 344). 



Fig. 342. — Crown-gall on Apple. 



