Plants as Affected by Excessive Water. 129 
221. Copious Waterings at Considerable Intervals 
are Preferable to frequent slight waterings. It should 
never be forgotten that air is as essential as water to the 
well-being of roots (90), and that the soil, however porous, 
requires occasional ventilation (94). A considerable quan- 
tity of water, poured upon the surface soil of a potted plant, 
in passing downward, not only thoroughly moistens the soil 
particles, but acts like a piston, forcing the vitiated air of 
the soil cavities ahead of it, and out through the drainage 
hole at the bottom of the pot, while fresh air enters from 
above as the surplus water passes out beneath. 
222. 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 narrow 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 
atmosphere 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 wooly covering of hairs are further examples of 
plants which require very little water’’.+ 7 
223. Excessive Watering sometimes Produces a 
Dropsical Condition (cedema) 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, abnor- 
mally distending their walls — sometimes even to bursting. 
+ Soraurer, Physiology of Plants, p. 207. 
