374: DIRECTION OF TRE GROWTH OF ROOTS. 



vigorous annual shoots, vhile the shaded branches become 

 languid and unhealthy. The motion of the ascending current 

 of sap appears therefore to be regulated by the ability to em- 

 ploy it in the tr^nk and branches of the tree; and this current 

 passes up through the alburnum, from which substance the 

 p f , buds and leaves spring. But the sap, which gives existence 

 enables the root to, and feeds the root, descends through the bark*: and 

 em'' kr?he"d». '^ *^^ operation of light give ability to the exposed branch to 

 scending sap. attract and employ the ascending or alburnous current of 

 sap, it appears not improbable, that the operation of pro- 

 per food and moisture in the soil, upon the bark of the 

 root, may give ability to that organ to attract and employ 

 the descending, or cortical current of sap ; and if this be 

 the case, an easy explanation of all the preceding pheno. 

 mena immediately presents itself. 

 Descent of roots A tree growing upou a wall, and unconnected with the 



from a tree on garth, will almost of nccessity grow slowly, and as it must 



p whII account- ' J a J 7 



cdfor. be scantily supplyed with moisture during the summer, it 



Avill rarely produce any other leaves, than those which the 

 buds contained, which were formed in the preceding year. 

 Some of the roots of a tree, thus circumstanced, will be less 

 well supplied with moisture than others, and these will be 

 first affected by drought : their points will in consequence 

 become rigid and inexpansible, and they will thence gene- 

 rally cease to elongate at an early period of the summer. 

 The descending current of sap will be then employed ia 

 promoting the growth and elongation of those roots only, 

 which are more favourably situate, and these comparatively 

 with other parts of the tree, will grow rapidly +. Gravi- 

 tation will direct these roots perpendicularly downward, 

 and the tree will appear to have adopted the wisest and best 

 plan of connecting itself with the ground ; and it will really 

 I have employed the readiest means of doing so, as effec- 



tively as it could have done, if it had possessed all the feel- 

 ings and instinctive passions and powers of animal life. The 



* Phil. Trans, 1809, p. 169: or Journal, vol.yxv, p. 118, 

 t We do not find here, however, " the proper food and mois- 

 ture," to " give ability to the root to attract and employ the de- 

 scending or cortical current of sap." C. 



subsequent 



i 



