Plants as Affected by Excessive Water. 139 



slower-growing ones. During the rest period (172), 

 plants should be given very little water. 



221. Some Species Require More Water than Others. 

 The native habitat of the plant is a partial guide 

 to the amount of water needed. Plants native to 

 arid regions, as the cacti and those from treeless, rocky 

 locations, require little water and are readily de- 

 stroyed by over watering. "Plants with narrow and 

 tough leaves, especially when the leaf-blade is ver- 

 tically placed, do not, as a rule, like much water; 

 plants with broad, leathery leaves prefer a damp 

 atmosphere to great moisture at the roots. Succulent 

 plants mth hard epidermal cells (leafless Euphorbias, 

 succulent Composites, Aloes and Agaves), and thin- 

 leaved plants with a strong wooly covering of hairs are 

 further examples of plants which require very little 

 water. ' '* 



222. Excessive Watering sometimes Produces a 

 Dropsical Condition (oedema) in the leaves of plants 

 under glass. This is most likely to occur in winter, 

 when sunlight is deficient, especially if the soil is kept 

 nearly or quite as warm as the air. Water accumulates 

 in the cells, abnormally distending their walls— some- 

 times even to bursting. An unnatural curling of the 

 leaves, with yellow spots and small wart-like excres- 

 cences on their surfaces, are some of the symptoms of 

 this trouble. Less water, increased light and reduced 

 bottom heat (362 a) furnish the remedy. 



Frenching, a disease that often attacks growing to- 

 bacco on excessively-wet clay soils, may be caused by 



* Sorauer, Physiology of Plants, p. 207. 



