INFLUENCE OF HEAT. 371 



which it hangs, and to the main trunk of the tree of 

 which it is a member. The tree represents a world, 

 every part exhibiting a mutual dependence. 



"The one red leaf, the last of its clan, 

 That dances as often as dance it can ; 

 Hanging so light and hanging so high, 

 On the topmost twig that looks up at the sky," 



is influenced by, and influences, the lowest root which 

 pierces the humid soil. Like whispering voices, the 

 trembling leaves sing rejoicingly in the breeze and 

 summer sunshine, and they tremble alike with agony 

 when the equinoctial gale rends them from the parent 

 stalk. The influences which pervade the whole, jnaking 

 up the sum of vital force, are disturbed by every move- 

 ment throughout the system ; a wound on a leaf is 

 known to disturb the whole, and an injury inflicted on 

 the trunk interferes with the processes which are the 

 functions of every individual leaf.* 



The consideration of the physical circumstances 

 necessary to germination and vegetable growth, brings 

 us acquainted with many remarkable facts. At a tem- 

 perature below the freezing point, seeds will not germi- 

 nate ; at the boiling point of water, a chemical change 

 is produced in the grain, and its power of germinating 

 is destroyed. Heat, therefore, is necessary to the de- 

 velopment of the embryo, but its power must only be 

 exerted within certain prescribed limits : these limits 

 are only constant for the same class of seeds, they vary 

 with almost every plant. This is apparent to every one, 

 in the different periods required for germination by the 

 seeds of dissimilar vegetables. 



The seed is placed in the soil; shade is always 



* See the very curious experiments of C. Matteucci. Traduit 

 et extrait du " Cimento." Archives des Sciences Physiques et 

 Naturelles; Quelques Experiences sur la Respiration des Plantes. 

 Nov. 1846. 



