242 TEXTBOOK OF BOTANY 



It is because of the steady evaporation of water from its 

 leaves that a garden plant whose roots do not reach far into 

 the soil wilts in dry weather ; the cells of its stem and leaves 

 lose so much water that they are no longer full and plump. 

 This explains why, to keep the plant alive at such times, we 

 must supply water to the soil ; and it shows us one reason 

 why the ground about the plant must be cultivated, so that 

 the upper layers of the soil shall be broken up and loosened 

 and thus prevented from giving off to the air the water which 

 the plant so badly needs. 



256. Necessity for the Water Current. — We know that 

 in seed plants the leaves ordinarily contain most of the 

 chlorophyl and manufacture m.ost of the carbohydrates. 

 The water which the leaves use in carbohydrate manufacture 

 comes from the roots, and this is one reason why the plant 

 must have a continuous series of conducting tissues leading 

 from the younger parts of the roots (where water is taken in) 

 to all parts of the leaves. The evaporation of water seems 

 to be important on warm days also in helping to cool the 

 leaves, which otherwise might be scorched by the sun's rays. 

 In this respect, the evaporation of water from leaves is useful 

 to the plant in the same way that the evaporation of perspira- 

 tion from our own bodies is useful to us. 



257. Dangers of too Rapid Evaporation. — The reasons 

 just given explain why there must be a constant supply of 

 water coming up from the roots ; still, in most cases the 

 amount of water that is necessary for these purposes is 

 probably small in comparison with the amount that actually 

 evaporates from the leaves. Not only is energy wasted in 

 thus bringing up to the leaves more water than is needed 

 for actual use ; but also the evaporation is a source of real 

 danger, because if the roots should not be able to supply 

 enough water to make up for what is lost from the leaves, 

 the leaves and stem would wilt. Such a loss that cannot 

 be made up is a serious matter ; water constitutes a large 



