DIRECTION OF PLUMULE AND EADIOLE. ' 353 



thus cause curvature outwards or inwards, the largest occupying the 

 convexity 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 neutralise the tendency to 

 incurvation on the upper side, which will therefore be directed either 

 upwards 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. 



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 up- 

 ward direction. If their positions are reversed they will become 

 twisted, so as to recover their natural positions. Henfrey remarks 

 that " so far as we are in a position to tell, there is some definite, 

 and as yet unknown, cause which makes the radicle first grow towards 

 the earth or other source of nourishment, which it penetrates by elonga^ 

 tion, a resisting point being offered by the weight of the seed or the 

 earth covering it ; and then, in its further growth downward, it 

 requires a point of resistance to be afforded by the adhesion of the 

 earth around the collar, ring, or neck of the root, since the elongation 

 takes place in the structures just above the point of the root, thus 

 exerting a pressure upwards and downwards, which if the upper part 

 of the root be kept free, and the weight of the plant balanced, wiU 

 cause the whole to rise bodily upwards. Thus, when seeds germinate 

 in damp moss lying upon a hard surface, the elongation of the root 

 will push the stem up through the moss, unless the root branches so 

 as to get fixed down by entanglement among the loose matter. We 

 may admit, therefore, that we are at present totally ignorant of the 

 cause of the direction taken by roots. All the notions hitherto 

 advanced having been purely speculative." 



The effect of light on the steni may be illustrated by the growth 

 of plants in circumstances where a pencil of light only is admitted on 

 one. side. Dr.'Poggioli of Bologna was the first who observed the 

 infiuence exercised by the rays of the spectrum in causing flection of 

 plants.. Experiments on this subject have been made by Payen, 

 Dutrochet, and Gardner. They consider the blue rays as those which 

 have the greatest eff'eot on the plumule. .Hunter observed, that if a 

 barrel filled -with earth, in the centre of which are some beans, was 

 rotated for several days horizontally, the roots pointed in a direction 



2 A 



