By Thomas Andrew Knight, Esq. 



115 



at considerable distances from each other, because the leaves 

 of the Pine Apple plants act less efficiently in the generation 

 of sap, in proportion as they are made to take a perpendi- 

 cular direction ; and this direction they are compelled to 

 take when they are laterally much shaded ; for the leaves 

 of this plant, like the stems of Potatoe plants, as I have re- 

 marked in the last Communication* which I had the honour 

 to address to this Society, are subject to the conflicting in- 

 fluence of gravitation *f and of light, the one labouring to give 

 a perpendicular, the other a horizontal direction to the leaves ; 

 and the comparative power of one agent encreasing as that 

 of the other decreases. 



I shall conclude the present Communication with an ac- 

 count of a very simple and efficient method of destroying the 

 different species of insect that infest the Pine Apple Plant, 

 which I have practised during the last two years with per- 

 fect success. Pine Apple Plants are not at all injured by 

 having water at the temperature of 150° of Fahrenheit's 

 scale thrown upon and into them, with a syringe. The 

 Mealy Bug does not appear to be injured by a single wash- 

 ing, or immersion for a short time in water of the above- 

 mentioned temperature ; but if the application be repeated 

 three or four times on as many successive days, it wholly 



* See page 406 of this Volume. 



f The influence of gravitation upon the forms of plants is still greater than I 

 have inferred in my Paper in the Philosophical Transactions upon that subject. 

 M. Duteochet having used very superior machinery to that employed by me, 

 discovered, that if a seed be made to revolve upon its own axis, and its axis of 

 rotation made to dip only a degree and a half below the horizontal line, the 

 roots will always take the descending, and the germs the ascending line, of 

 that axis. 



