DIRECTION OF PLUMULE AND RADICLE. 299 



thinks that the direction of stem and roots may be traced to the state of 

 the tissues. When a branch is horizontal, the fluids gravitate towards 

 the lower side ; a vigorous growth takes place there; the tissues enlarge, 

 and, by increasing more than those on the upper side, an incurvation 

 is produced, the convexity of which looks downwards, and thus the 

 extremity of the branch is directed upwards. Again, in the root the 

 increase takes place by the extremity, and the fluids by their gravity 

 cause this to retain always a descending direction. A similar explana- 

 tion is given by Dodart. Dutrochet refers the phenomena to endos- 

 mose, which varies in its effects according to the comparative size of 

 the cells in the centre and circumference of an axis. In young stems 

 with large pith, the central cells are larger, and they diminish towards 

 the circumference ; whereas in roots, accordiag to him, the diminution 

 takes place in the reverse manner. Large cells distend more rapidly 

 than small ones ; and, according to their position in the axis, will thus, 

 cause curvature outwards or inwards, the largest occupying the con- 

 vexity of the arch, the smallest the concavity. When a branch or root 

 is laid horizontally, the force of endosmose is weakened on the lower 

 side, and, consequently, will cease to neutralize the tendency to incur- 

 vation on the upper side, which will therefore be directed either up- 

 wards or downwards, according to the position of its layers of small 

 cells, in the case of a branch with large central cells, curving upwards ; 

 and in the case of a root with larger hemispherical cells, downwards. 



625. These explanations do not appear, however, to be altogether 

 satisfactory. It is known that the stem is directed upwards, the root 

 downwards, but, as yet, physiologists have not been able to ascertain 

 the laws which regulate them. The tendencies of the root and stem 

 are not easily counteracted. When a seed is planted in moist earth, 

 and suspended in the air, the root will, in the progress of growth, leave 

 the earth and descend into the air in a perpendicular direction, while 

 the stem will pass through a quantity of moist earth in an upward 

 direction. If their positions are reversed they will become twisted, so 

 as to recover their natural positions. 



626. The effect of light on the stem may be illustrated by the 

 growth of plants in circumstances where a pencil of light only is 

 admitted on one side. Experiments on this subject have been made 

 by Payen, Dutrochet, and Gardner. They consider the blue rays as 

 those which have the greatest effect on the plumule. Hunter observed, 

 that if a barrel filled with earth, in the centre of which are some 

 beans, be rotated for several days horizontally, the roots pointed in a 

 direction parallel to the axis of rotation. Knight* put Mustard seeds 

 and French beans on the circumference of two wheels, which were 

 put in rapid motion, the one in a horizontal, and the other in a vertical 

 manner; and he found that, in the former, the roots took a direction 



* See Knight's Horticultural Papers, London, 1841, p. 124. 



