322 PEINCIPLES OF GAKDENING. [CH. IX. 



no more power than have animals to escape from the 

 injurious excitement occasioned by being compelled to 

 live for any protracted time in a temperature uncon- 

 genially elevated. In such a temperature, youthful 

 and growing animals are stimulated to an excessive 

 rapidity of growth, so attenuating, that nothing but 

 removal to a colder climate can preserve them from 

 premature death ; and the same phenomena attend 

 upon plants. These, over-excited by heat, acquire 

 rapidly an unnaturally elongated growth, attended 

 by a weakness of texture, that hastens them to decay, 

 unless checked by a gradual reduction of tempera- 

 ture. The roots in such a heat absorb water with 

 unnatural rapidity, and this is commensurately 

 hurried through the sap vessels of the stem and 

 branches, so that the over wateiy sap arrives at the 

 leaves much too fast for them to elaborate it suffici- 

 ently, though an extra effort is made by preternatur- 

 ally enlarging the leaves. The water transpired is 

 excessive, but very little carbonic acid is inhaled, and 

 consequently the quantity of carbon assimilated is 

 very deficient. The whole structure of the plant is 

 therefore wateiy and weak ; and if a supply of water 

 to the roots is withheld but for a few hours, the leaves 

 mther and shrivel past revival. These organs not 

 only lose the power to decompose carbonic acid, but 

 also to decompose water, though the light to which 

 they are exposed be the brightest sunshine ; and 



