266 APPLICATION OF PRINCIPLES. 



of the sedimentary matter of lignification,* and to 

 be induced by want of motion and excessive expo- 

 sure of the leaves and branches to the sun. The effect 

 of the latter is to inspissate all the juices, and to pro- 

 mote their formation ; while the former increases the 

 evil by not keeping the fluids in rapid circulation : 

 just as we know that a slow stream, from a muddy 

 source, deposites its impurities much more copiously 

 than a rapid stream. As this evil arises out of the 

 operation of training, and seems to be inseparable 

 from it, there will be no expectation of a remedy 

 being discovered. 



The increase of the saccharine quality of fruit is by 

 no means an advantage in all cases ; it improves the 

 peach, the nectarine, the pear and the plum, in which 

 sweetness is the great object ; but it deteriorates the 

 apple and the apricot, which are chiefly valued for 

 their peculiar mixture of acidity and sweetness. 



The protection received in the spring by trees 

 trained upon walls exposed to the sun, while it 

 advances the period of flowering, at the same time 

 causes it to take place at a season when they are not 

 sufficiently secure from spring frosts ; and hence the 

 necessity of protecting such plants artificially by 

 coping, screens, bushes, curtains, and other contri- 

 vances. It is on this account that the utility of flued 

 walls is so much diminished, and that they are found, 

 in practice, more valuable for ripening wood in 

 autumn, than for guarding blossoms in the spring. 



* See Introduction to Botany, ed. 8, p. 8 



