70 THE MECHANICAL FUNCTIONS. 



spongioles in absorbing fluids I shall have occasion 

 to speak when treating of nutrition : but as the 

 roots exercise a mechanical as well as a nutrient 

 office, we should here consider them in the light of 

 organs adapted to procure for the plant a perma- 

 nent attachment to the soil, upon which it is wholly 

 dependent for its supply of nourishment. It is 

 scarcely necessary to point out how effectually 

 they perform this office. Our admiration cannot 

 fail to be excited when we contemplate the manner 

 in which a large tree is chained to the earth by its 

 powerful and widely spreading roots. By the firju 

 hold which they take of the ground, they exert the 

 most effectual resistance to the force of the w^inds, 

 which, acting upon so large a surface as that pre- 

 sented by the branches covered with dense foliage, 

 must possess an immense 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 different distances in order to procure the requi- 

 site nourishment, so the stem grows upwards for 

 the purpose of obtaining 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, occupying the surface of the cylinder. 

 Of all the possible modes of disposing a given 

 quantity of materials in the construction of a 

 column, it is mathematically demonstrable that this 



