70 THE MECHANICAL FUNCTIONS. 



the most effectual resistance to the force of the winds, which, 

 acting upon so large a surface as that presented by the 

 branches covered with dense foliage, must possess an im- 

 mense mechanical power. 



The principal seat of the vitality of a plant is the part 

 which intervenes between the root and the stem. Injuries 

 to this part are always fatal to the life of the plant. 



As the roots penetrate downwards into the earth to dif- 

 ferent distances in order to procure the requisite nourish- 

 ment, so the stem grows upwards for the purpose of obtain- 

 ing for the leaves and flowers an ample supply of air, and 

 the influence of a brighter light, both of which are of the 

 highest importance to the maintenance of vegetable life. 

 The stems of the grasses are hollow tubes; their most solid 

 parts, which frequently consist of a thin layer of silex, oc- 

 cupying the surface of the cylinder. Of all the possible 

 modes of disposing a given quantity of materials in the con- 

 struction of a column, it is mathematically demonstrable that 

 this is the most eff'ective for obtaining the greatest possible 

 degree of strength.* 



The graceful continuous curve with which the stem of a 

 tree rises from the ground, is the form which is best calcu- 

 lated to give stability to the trunk. Evidence of express 

 mechanical design is likewise afforded by the manner in 

 which the trunk is subdivided into its branches, spreading 

 out in all directions, manifestly with a view to procure for 

 the leaves the greatest extent of surface, and thus enable them 

 to receive the fullest action of both light and air. The 

 branches, also, are so constructed as to yield to the irregular 

 impulse of the wind, and again, by their elasticity, to return 

 to their natural positions, and by these alternate inflexions 

 on opposite sides, to promote the motion of the sap in the 

 •vessels and cellular texture of the liber and alburnum. No- 

 thing can exceed the elegance of those forms which are pre- 



* Galileo, the most profound philosopher of his age, when interrogated 

 by the inquisition as to his belief in a Supreme Being-, repUed, pointing to 

 a straw on the floor of his dungeon, that from the structure of that object 

 alone he would infer with certainty the existence of an intelligent Creator. 



