THE PLANT AS AFFECTED BY WATER 131 



220. Rapidly-growing plants require more water and 

 are less liable to suffer from over-watering than slower- 

 growing ones. During the rest period (172), plants should 

 be given very little water. 



221. Water requirement of plants. — 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 destroyed by over-watering. " Plants with nar- 

 row and tough leaves, especially when the leaf-blade is 

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

 plants with broad, leathery leaves prefer a damp atmos- 

 phere to great moisture at the roots. Succulent plants 

 with hard epidermal cells (leafless Euphorbias, succulent 

 Composites, Aloes and Agaves), and thin-leaved plants 

 with a strong woolly covering of hairs are further examples 

 of plants which require very little water." ^ 



222. Dropsical condition. — Excessive watering some- 

 times 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 accu- 

 mulates in the cells, abnormally distending their walls — 

 sometimes 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) furnish the remedy. 



Frenching, a disease that often attacks growing tobacco 

 on excessively-wet clay soils, may be caused by undue ab- 

 sorption of water by the roots. The leaves of affected 



1 Sorauer, "Physiology of Plants," p. 207. 



