the Direction of the Growlh of Boots. 423 



the instinctive faculties and passions of animal life; and as 

 I concluded before I made the experiment that they would 

 do, under the guidance of nuich more simple laws, whose 

 mode of cpcraiing I shall endeavour to explain. 



Whatever be the machinery by which ihe sap of trees is 

 raised to the extremities of their branches, it is obvious that 

 this machinery is first put into acion bv the stems and 

 branches, and not bv the roots : for the graft or bud, when- 

 ever it has become fully united to the stock, wholly rey;u- 

 lates the season and temperature in which the sap is io be 

 put in motion, in perfect independence of the habits of the 

 stock ; whether those be late or early, if all ihe branches 

 of a tree, exclusive of one, be nuich shaded by contiguous 

 trees*, or other objects, the branch which is exposed to tlie 

 light attracts to itself a lar^e portion of the ascendins sap, 

 which it employs in the formation of leaves and vigorous 

 annual shoots, whilst the shaded branches become languid 

 and unhealthy, 'i'hc motion of the ascending current of 

 sap appears therefore to be regulated bv the ability to em- 

 ploy it in the trunk and branches of the tree; and this cur- 

 rent passes up through the alburnum, from which substance 

 the buds and leaves spring. But the sap which gives ex- 

 istence to, and feeds the root, descends through the bark f: 

 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 em- 

 ploy the descending or cortical current of fap ; and if this 

 be the case, an easy explanation of all the preceding phse- 

 nomena inniicdiatelv presents itself. 



A tree growing upon a wail, and unconnected with the 

 earth, will almost ot necessity grow slowly ; and as it must 

 be scantdy supplied with moisture during the summer, it 

 will rarelv produce any other leaves than those which the 

 buds contained which were formed in the |)receding year. 

 Some of ihe 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 mexpansible, and they will thence ge- 

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

 The descending current of sap will be then employed in 

 promoting the growth and tlongalion of tho?c roots only 

 which are more favourably situated, and those, compara* 



. ♦ Pliil. 'Iran*. 1805 and 1809, p. 8. f Phil. Trans, part i p. I. 



^ D d 4 t'lvely 



J '. 



