Plants as Affected hy Excessive Water. 135 



as the cacti and those from treeless, rocky locations, re- 

 quire little water and are readily destroyed by over- 

 ^-atering. '_' Plants with narrow and tough lea^'cs, espe- 

 cially when the leaf-blade is vertically placed, do not, as 

 a rule, like much water; plants with broad, leathery 

 leaves prefer a damj) atmosphere to great moisture at the 

 roots. Succulent plants with hard epidermal cells (leaf- 

 less Euphorbias, succulent C'(miposites, Aloes and Agaves), 

 and thin-leaved plants ^-ith a strong wooly covering of 

 hairs are further examples of plants which require very 

 little water."* 



223. Excessive Watering sometimes Produces a Drop- 

 sical Condition (oedema) in the leaves of plants under 

 glass. This is most likely to occur in winter, when sun- 

 light is deficient, especially if the soil is kept nearly or 

 quite as wai-m as the air. Water accumulates in the 

 cells, abnormally distending their "nails — sometimes even 

 to bursting. An unnatural curling of the leaves, with 

 yellow spots and small wart-like excrescences on their 

 suifaces, 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 tobacco 

 on excessively-wet clay soils may be due to the undue 

 absorption of water by the roots. The leaves of affected 

 plants grow narrow, and are thick, fleshy and crinkled. 

 If the plants are pulled sufficiently to break the tap-root, 

 before the disease has progressed too far, they often 

 recover. 



224. Water-Sprouts (sap-sprouts, gormands) on fruit 

 trees are sometimes due to an excess of water in the soil. 



* Sorauer, Physiology of Plants, p. 207. 



