342 SPFXIAL DIRECTIONS AND 



stem out of the perpendicular line ; which side, thus more actively 

 nourished, would grow more vigorously than the upper, and so cau.-e 

 the stem to turn upwards. To show how baseless this ingenious 

 hypothesis is, we have only to remember, on the one hand, that the 

 fluid contents of the cells of plants arrange themselves in obedience 

 to other forces than gravity, and freely rise against its influence to 

 the summit of the loftiest trees, so that gravity could establish no 

 difference within the diameter of a germinating stem ; and on the 

 other, that the root in germination, if fixed upon its surface, will pen- 

 etrate a fluid of greater weight than itself, such as mercury. More- 

 over, Schultz and Mohl have shown that, by careful management in 

 reversing the ordinary conditions, — as by germinating seeds in 

 damp 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, — the ordinary direction of the organs could be reversed, 

 the roots turning upwards into the dark and damp moss, and the 

 stems downward into the light. This would prove that light has 

 more effect than gravitation, or any other imaginable influence of the 

 mass of the earth. Yet, — what shows that there is some real relation 

 between the direction assumed by the plant and the earth, — stems 

 which grow in complete darkness always point to the zenith, as is 

 seen in the shoots of vegetables in perfectly dark cellars, and in the 

 elongated, constantly upright stemlet of germinating seeds too deeply 

 buried to receive any light before they reach the surface of the soil. 



GG8. The influence of a mass in some way analogous to attrac- 

 tion is also observed in the germination of the Mistletoe. Its form- 

 ing root turns regularly to the trunk or branch upon which it is. 

 parasitic, just as those of ordinary plants turn to the earth. And 

 that it is the mass and not the quality of the body which determines 

 the direction, is seen when germinating seeds of the Mistletoe are 

 fixed close to the surface of a cannon-ball : all the roots as they 

 grow point to its centre and advance to its surface, just as they do 

 to the branch of a tree which they penetrate. 



6G9. When the stem has emerged from the earth into the light of 

 day, this exerts a controlling influence over its direction. Young 

 and green stems always tend to expose themselves as much as possi- 

 ble to the light, and bend, very promptly when delicate, towards the 

 most illuminated side, as is well observed when plants are raised in 

 an apartment lighted from a single aperture : and consequently in 

 the open air, being equally illuminated on all sides, they grow up- 



