CHAPTER II 
DISEASED CONDITION OF PLANTS DUE TO 
ENVIRONMENTAL FACTORS—LIGHT, HEAT, 
HUMIDITY, SOIL, ETC. 
THE various factors which comprise a plant’s environ- 
ment have an important bearing upon its health; and 
while certain conditions have a detrimental effect upon 
its power of resistance to disease, others actually induce 
a diseased condition. 
Light 
_ Itiscommon knowledge that plants grown in darkness 
become elongated, slender, whitish in colour, and im- 
mediately wilt when exposed to the heat and light of an 
ordinary glasshouse. 
Similar effects are produced in a lesser degree when 
glasshouse plants are exposed to long periods of dull, 
cloudy weather. 
For instance, plants grown during November and 
December possess imperfectly coloured weak leaves, 
with slender petioles and badly developed stems. The 
weak light is insufficient to mature the mechanical and 
resistant tissues, and consequently such plants wilt in 
the bright sun of early spring. This physiological 
wilting is well known to growers of early cucumbers, 
and indeed may appear later in the year if after long 
periods of dull weather the plants are exposed to high 
temperatures and bright light. Such plants recover 
during the night, but wilt again during the day. The 
trouble becomes accentuated in weak plants grown in 
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