THE NURSERY. 315 



buried and the soil too rich at the same time, they 

 will jointly injure the quality of the tree. 



Cultivators sometimes forget (and it is often an 

 unfortunate forgetfulness,) that the healthy con- 

 dition of a plant does not depend on the soil, the 

 moisture, or the heat ; that it does not depend 

 upon all three jointly, or on the proportions that 

 they bear to each other. To that part of the 

 plant which naturally lives in the air, there must 

 be light ; and although their artificial heat with- 

 out light may do for those roots that are naturally 

 under ground, it is extremely doubtful whether 

 any substitute can be found for the beams of the 

 sun. So, if there is artificial heat applied to the 

 leaves, its action will be imperfect, and the qua- 

 lity of the plant deteriorated if there is not the 

 light of the sun along with it. 



The soil, the humidity, the air, the heat, and 

 the light, must, like all causes which work jointly 

 in producing an effect, be duly proportioned to 

 each other; and when, in any combination of 

 that kind, there is any one of the causes over 

 which we have no control, we must regulate our 

 measure of all the others by that. Now the light 

 and heat of the sun are the only causes of the 

 growth of vegetables which are without the con- 

 trol of man as to quantity, and the light is most 

 exclusively so. We have not the smallest power 

 over it, either in respect of duration or of inten- 

 sity. Perhaps something might be done by means 

 of mirrors, but they have not been tried, and 

 they could not be used on the great scale. In- 

 deed it is probable that any attempts to increase 

 the intensity of light by artificial means would do 

 mischief rather than good. On sunny days, any 

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