THEIR SPECIAL DIRECTIONS. 341 



obliquely upwards and inwards, in obedience both to the centrifu- 

 gal force and the power of gravitation, acting at right angles to 

 each other. That light is the chief cause of the upward direction 

 of the stem, while it is avoided by the roots, appears from a recent 

 experiment by Professor Schultz, of Berlin ; who reversed the nat- 

 ural condition, by causing seeds to germinate in Moss, so arranged 

 that the only light they could receive was reflected from a mirror, 

 which threw the solar rays upon them directly from below ; in 

 which case he found that their roots were sent upward into the 

 Moss, contrary to the ordinary direction, and their stems down- 

 ward towards the light. 



650. The Mistletoe obeys the attraction of the trunk or branch 

 upon which it is parasitic (134), just as ordinary plants obey the 

 attraction of the earth ; its roots penetrating towards the centre, 

 while the stems grow perpendicular to the surface of the branch, 

 and are therefore placed in various positions as respects the earth. 

 When the germinating seeds of the Mistletoe were glued to the 

 surface of a cannon-ball, all the radicles were found to be directed 

 toward its centre. A well-devised experiment made by Dutrochet 

 goes to show, that the direction of the radicle to the adjacent body 

 (and consequently of the germinating root generally towards the 

 earth's centre) is not the result of the immediate attraction of the 

 adjacent body, or of the earth, but is a spontaneous movement due 

 to some internal, vital cause, put in/ action by the exterior influ- 

 ence. He mounted the seed of a Mistletoe upon one extremity of 

 a very delicately balanced needle, which would turn with the 

 slightest force, and placed it at the distance of half a line from the 

 surface of a large cannon-ball. In germination the radicle directed 

 its point to the ball, and soon came into contact with the surface ; 

 but that end of the needle had not moved in the slightest degree 

 towards the ball, as it would have done from a mere exterior at- 

 traction. 



651. When the stem has emerged from the earth, it tends to 

 expose itself as much as possible to the light, the growing parts 

 always turning towards the side most strongly illuminated ; as is 

 observed when a plant is placed in an apartment lighted from a 

 single aperture. This is mechanically accounted for by De Can- 

 dolle, on the supposition, that, as the side upon which the light 

 strikes will fix most carbon by the decomposition of carbonic acid, 

 so its tissue will become more solid than the shady side, and there- 



29* 



