THE AGENTS WHICH AFFECT PLANTS. 



13 



Water is very necessary and very refreshing to the 

 leaves of plants, to wash away dust and dirt from them, 

 and keep their pores in healthy action ; besides checking 

 any extravagant drain on their resources in dry weather. 

 Rains and dews are beneficial in this way, for the most 

 part. And in artificial watering, whether given to plants 

 in the open ground or in pots, syringing over the leaves 

 will be an important addition, without which common 

 watering at the roots would be of comparatively little 

 avail. But it should be seen that the water, however 

 applied, is not of an injurious nature, and does not con- 

 tain deleterious matter. 



5. — Electricity, 



In the absence of any definite knowledge of this myste- 

 rious power, it can only be mentioned as a thing that acts 

 decidedly and strongly upon plants. There can be no 

 doubt that it promotes healthiness, when present in only 

 its ordinary condition and quantity. But it also seems, at 

 least, to occasion disease, and to be in some sort produc- 

 tive of what are popularly termed " blights," which are 

 sometimes in no way attributable to insects. How far it 

 may go, in its usual state, towards composing or upholding 

 vegetable life, it is impossible to say. ^'either can it be 

 determined, by any means at present known or under- 

 stood, to what extent (if at all) it has been productive of 

 the disease which has so unhappily become notorious as 

 the potato blight," though this is most commonly 

 ascribed to atmospheric influences. But as the further 

 discussion of this principle could not tend to any positive 

 practical result, it may be dismissed with a simple refer- 

 ence to the known potency of its action on vegetable life. 



Q.— Weather, 



The great variations of the weather consequent on 

 atmospheric changes, and forming the climate of a dis- 

 trict, exert a powerful agency upon plants, and recpiire to 

 be well considered and studied. The barometer, thermo- 

 meter, and even the hygrometer, to measure the heat and 

 moisture and calculate the changes of the atmosphere, 



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