36 BIOLOGY OF PLANTS. 



II. Eicitahility in vegetables indicates the existence of a 

 vital principle, and is one of its distinguishing properties. It 

 is a capacity of being acted upon by natural and artificial stim- 

 ulants, such as light, heat, electricity, manures and saline com- 

 pounds. 



1. Light. The stimulating influence of light upon the 

 leaves and blossoms of vegetables, cannot have escaped the 

 most common observation. The leaves turn their upper sur- 

 faces to the sun, and the blossoms of many plants close during 

 the night, called the deep of plants, and open only when sub- 

 jected to the influence of that agent. Plants that grow in the 

 shade are not so highly colored nor so vigorous, as those which 

 are exposed to the light, and generally the branches and the 

 fruit are the most vigorous on the south side of the tree. The 

 ripe ears of grain generally lean toward the south. The 

 branches grow in the direction of a crevice, in the wall of a 

 ceflar, through which light is admitted. These, and a great 

 variety of phenomena prove the existence of a vital power, or 

 capacity of being excited by the agency of light. 



2. Heat exerts a powerful influence upon the functions of 

 plants. This is seen in the germination of the seed, a certain 

 temperature being requisite to develope the germ, and enable 

 it to throw out roots and stalks. In the production of leaves, 

 flowers and fruit, each development depends, to some extent, 

 upon the degrees of heat which are applied, — hence the vari- 

 ous means which are employed to increase or diminish the 



roots and leaves, gases and liquids arc converted into solids, and by a 

 well known law in such cases, heat is evolved; the insensible heat be-, 

 comes sensible ; on the same principle, as the temperature of tlie air di- 

 minishes, and the sap beg-ins to freeze, a quantity of heat is evolved. 

 The power of resisting heat is partly accounted for on the principle 

 that the plant transpires a large quantity of water through the leaves, 

 and as the heat increases, the quantity of water is increased ; by the 

 conversion of the water into vapor, a large quantity of caloric becomes 

 insensible, the external heat is thus taken up, and the temperature of 

 the plant remains below that of the surrounding medium. 



