AND THE DISEASES OF PLANTS. 297 



object aimed at ; but if, on the contrary, it be timber, we 

 must promote the descent of the cambium into the trunk, 

 instead of endeavoring to detain it in the branches. 



Gentle exercise is, however, generally advantageous in 

 the vegetable economy, and promotes the circulation of 

 the juices : while violent motion occasions either exhaus- 

 tion or fever. Hence the objection to props and espaliers; 

 which we have already noticed. Boisterous winds are also 

 mechanically injurious to trees, rending their branches, 

 and sometimes tearing up their roots from the soil. 



Plants are affected by the nature of the atmosphere in 

 which they grow. There is nothing more prejudicial to 

 them than smoke. 



Emily. I am surprised at that ; for smoke, you have 

 told us, consists of small particles of carbon which have 

 escaped combustion ; and carbon, you know, is the favo- 

 rite food of plants. 



Mrs. B. The particles of smoke, though apparently 

 so small to our senses as scarcely to be distinguished when 

 separate, are mountains compared to the very minute 

 subdivision which matter must undergo, in order to enter 

 into the vegetable system. Smoke may clog the pores 

 of plants, but can never gain admittance through them. 



Emily. But smoke is always accompanied by a current 

 of hot air, which must be strongly impregnated with car- 

 bonic acid ; and in this state the carbon is so minutely 

 subdivided as to be quite invisible, and, I suppose, suffi- 

 ciently so to enter the pores of plants. 



Mrs. B. If plants absorb carbonic acid by their leaves, 

 or any part exposed to the air, it can be but in very small 

 quantities. Under common circumstances, it enters into 

 their system, only by their roots ; it is their leaves which 

 decompose it. Carbonic acid gas is as prejudicial to 

 plants externally as it is to animals; for plants, under a 

 receiver containing carbonic acid, die in the course of a 

 few hours. Azote and hydrogen do not appear to be in- 

 jurious to plants, unless in such quantity as to diminish 

 the proportion of oxygen in the atmosphere, which their 

 vegetation requires. 



1601. How is the production of fruit or of timber affected by the cam- 

 bium'? 1602. What is said of gentle exercise 1 ? 1603. And of bois- 

 terous winds'? 1604. How does smoke affect plants'? 1605. Why 

 is it not beneficial, as carbon is a favorite food of vegetables'? 1606. 

 Why is not the carbonic acid in smoke favorable to vegetation 1 1607 

 What is azote and hydrogen 1 



