^ INFLUENCE OF GRAVITY. 99 



power which all bodies, in masses, possess of attracting each 

 other. Gravity causes a stone and all heavy bodies throuTi 

 into the air to fall to the earth. The direction of this force 

 is towards the centre of the body ; hence, all bodies falling to 

 the earth, if not arrested at its surface, would pass on direct- 

 ly to its centre. This force is the cause of the pressure of 

 the atmosphere, and of water, as well as of their motion. Wa- 

 ter, by the force of gravity, falls from the clouds, penetrates 

 the earth, and hurries to the ocean in streams and rivers. 



The principal effect of gravity upon the functions of vege- 

 tables, is the influence it exerts upon the direction of their 

 roots and branches. This influence was proved by the ex- 

 periments of Mr. Knight, who fixed some seeds of the garden 

 bean on the circumference of two wheels, the one made to 

 revolve rapidly in a horizontal direction, and the other in a 

 vertical direction. The beans were supplied with the requi- 

 site conditions for germination, and although the revolutions 

 on the vertical wheel were two hundred and fifty per minute, 

 and those on the horizontal, one hundred and fifty, the beans 

 all grew. The roots in the vertical wheel pointed in the di- 

 rection of the radii, and the stalks towards the centre where 

 they soon met. In the horizontal wheel the centrifugal force 

 conflicted with that of gravitation, and caused the stems or 

 stalks to meet, in the form of a cone, over the centre of revo- 

 lution, while the roots took an opposite direction. There 

 can be no doubt, that gravity exerts considerable influence on 

 the direction of the roots and stems of plants, and yet the rea- 

 son why the seed takes root downward, and bears fruit up- 

 ward, must be attributed to the laws of vitality * 



II. Cohesion. Cohesive attraction holds the parts of bo- 

 dies together. This is shown when two leaden bars are 

 scraped smooth, and pressed together ; they will adhere with 

 a force proportioned to the closeness of their contact. If that 

 is perfect, the bar will yield in any other part as easily as at 



* See Davy's Ag. Chemistry. 



