160 ON THE DIRECTION 



the bottom of the inverted pot, in sufficient quantity to keep the mould 

 moderately moist ; and the pots being suspended from the roof of a 

 forcing-house, the seeds soon vegetated. 



In former experiments*, wherever the seeds were placed to vegetate 

 at rest, the radicles descended perpendicularly downwards, in whatever 

 direction they were first protruded ; but under the preceding circum- 

 stances they extended horizontally along the surface of the mould, and in 

 contact with it ; and in a few days emitted many fibrous roots upwards 

 into it : just as they would have done, if guided by the instinctive facul- 

 ties and passions of animal life ; and as I concluded before I made the 

 experiment that they would do, under the guidance of much more simple 

 laws, whose mode of operating I shall endeavour to explain. 



Whatever be the machinery by which the sap of trees is raised to the 

 extremities of their branches, it is obvious that this machinery is first 

 put into action by the stems and branches, and not by the roots : for 

 the graft or bud, whenever it has become fully united to the stock, wholly 

 regulates the season and temperature, in which the sap is to be put in 

 motion, in perfect independence of the habits of the stock ; whether 

 those be late or early. If all the branches of a tree, exclusive of one, 

 be much shaded by contiguous trees-f, or other objects, the branch which 

 is exposed to the light attracts to itself a large portion of the ascending 

 sap, which it employs in the formation of leaves and vigorous annual 

 shoots, whilst the shaded branches become languid and unhealthy. The 

 motion of the ascending current of sap appears therefore to be regulated 

 by the ability to employ it in the trunk and branches of the tree ; and 

 this current passes up through the alburnum, from which substance the 

 buds and leaves spring. Bat the sap which gives existence to, and feeds 

 the root, descends through the barkj : and if the operation of light give 

 ability to the exposed branch to attract and employ the ascending or 

 alburnous current of sap, it appears not improbable that the operation 

 of proper 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 cor- 

 tical current of sap ; and if this be the case, an easy explanation of all 

 the preceding phenomena immediately presents itself. 



A tree growing upon a wall, and unconnected with the earth, will almost 

 of necessity grow slowly, and as it must be scantily supplied with moisture 

 during the summer, it will 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 



* See above, No. VII. f Ibid. No. VI. and XII. I See the last Paper. 



