'black rot' of cabbages 807 



with a single- cilium. The bacterium frequently enters through 

 the water-pores which are present on the teeth at the edges of 

 the leaves, and most easily gain admittance in damp, warm 

 weather when drops of water are excreted by these pores and 

 stand like dew-drops along the leaf-margins. 



Slugs and the larvae of insects carry them from the soil, and 

 from one infected plant to another, the bacteria eventually 

 entering into the vascular tissues exposed at the parts gnawed 

 and bitten by these pests. 



The bacteria live and multiply in the alkaline solutions present 

 in the vessels or tracheae of the vascular bundles, and do not 

 invade the parenchymatous tissue of the leaf. The vessels 

 become blocked with a brown substance which prevents the 

 proper conduction of water, hence the withering of the diseased 

 leaves. 



It is advisable to remove and burn diseased plants as soon as 

 these are noticed, and on ground where the trouble has been 

 prevalent cruciferous plants should not be grown for some time. 



An organism named Pseudomonas destructans has been de- 

 scribed recently by Potter, and to it is ascribed a 'white rot' 

 of turnips. The leaves of the infected plants turn yellow and 

 droop, and the interior of the ' root ' in 2 or 3 weeks becomes 

 pulpy and rotten. 



The bacterium softens the cell-walls and secretes an enzyme 

 which dissolves the middle lamella of the cells of the turnip 

 ' root.' The protoplasm separates from the cell-walls and turns 

 brown, possibly as the result of the toxic action of oxalic acid 

 which is produced by the bacterium. 



The organism appears to gain an entrance into the tissues of 

 the turnip through wounds. 



15. Another disease directly caused by a bacillus is prevalent 

 among melons and cucumbers and other species of Cucurbitaceae. 

 The leaves of affected plants droop and change from a bright 

 to a dull green colour. 



