228 DISEASE IN PLANTS. 



Tumescence. It occasionall)' happens that 

 herbaceous parts of plants pass into a condition 

 of over-turgescence from excess of water in the 

 tissues, an abnormal state which indicates patho- 

 logical changes resulting from various causes, 

 often not evident and therefore regarded as 

 internal. Such disease was formerly termed 

 CEdema or Dropsy. This disease is frequently 

 due to the excessive watering of pot plants with 

 large root systems and deficient foliage, in hot- 

 houses with a saturated atmosphere : it is, therefore, 

 primarily referable to diminished transpiration. 

 It can sometimes be brought about by covering 

 potato plants, for instance, with a bell-jar in 

 moist, hot weather ; and this, and the prevalence 

 of the disease in hot-houses as compared with 

 plants grown out of doors, point to the above 

 explanation. Similar phenomena do occasionally 

 occur out of doors in hot, moist situations 

 or during wet seasons, however, and the watery 

 shoots of rank vegetation are merely particular 

 cases of the same class. Moreover, the well- 

 known tendency to succulence of sea-side varieties 

 of plants which have thin herbaceous leaves when 

 growing inland, points to the action of the 

 environment in these matters, excess of salts 

 being no doubt one factor in such cases. 



Ranhtess affords another example where super- 

 fluity of water is concerned, though it does not 

 involve simply this, because the plant may also 

 contain excessive quantities of nitrogenous and 

 mineral matters taken up by the roots. 



