244 BACTERIAL ORIGIN OF GUMS OF ARABIN GROUP, 



lapse of about a month not only could no slime be formed, but 

 the cells refused to grow. A few experiments showed that the 

 reason for this was that the surface of the agar had become dry 

 during the interval that passed between the sloping of the tube 

 and its utilisation. The medium must therefore be recently 

 prepared if a production of slime is desired. 



The j)Ossihility of curing the gimi-flxix of trees. — In considering 

 the possibility of being able to prevent or check the formation of 

 gum by the bacteria in the tissues of plants, it is evident that we 

 have to deal with organisms that flourish in poor substrata. A 

 small quantity of a suitable sugar (^%) and of a nitrogenous sub- 

 stance (2^%) ^^® enough for the bacteria to produce half the 

 maximum amount of gum. Rich substrata rather tend to pre- 

 vent gum-formation; this has been seen in the effect of sugars 

 and salts. It has been noted that gum is found upon trees 

 in unhealthy surroundings. Such conditions would reduce 

 the composition of the sap below the normal, and while the 

 growth of the tree was hindered the growth of the bacteria 

 would be accelerated. The influence of temperature is very pro- 

 nounced, and a reference to the quantity of slime produced at 

 different temperatures is enough to show how the formation of 

 gum is increased by a cold season or by the presence of the tree 

 in a damp, cold hollow. In the latter cases the drainage will 

 have an influence in lessening the vitality of the tree. In all 

 questions of invasion of plant tissues by bacteria it is a rule that 

 a healthy plant will overcome the attacks of the organisms, and 

 it probably does this by itself utilising the products of its own 

 metabolism, thus starving the bacteria, and also by maintaining 

 the acidity of its juices. The success of the plant is also pro- 

 bably influenced by the small numbers of invading bacteria, for a 

 large infection would enable the organisms to get a hold which 

 would lead to the defeat of the plant. A mass of bacteria will 

 secrete more byproducts than a few cells, and these being localised 

 in the host plant may injuriously affect the cells in the locality, 

 and thus enable the bacteria to establish themselves. 



