BOTANY. 



4-*-*- 



ON THE DIRECTIONS ASSUMED BY PLANTS. 



WE find, in the Philosophical Transactions for 1848, an interesting 

 paper by Professor Macaire, of Geneva, in Switzerland, on the direc- 

 tions assumed by plants in growing. The author first examines expe- 

 rimentally into the causes of the curling up of the tendrils, which 

 Knight endeavored to explain by the unequal action of the light on both 

 sides of the tendril, and which was attributed by De Candolle to the 

 obstacle afforded to vegetation by the contact of the leaf-stalk with the 

 body adhered to, on the side where it touches. Prof. Macaire selected, 

 to experiment upon, a common Swiss weed, and he found, that, when 

 the tendril is touched by any solid body whatever on a point of the sur- 

 face not too far from the extremity, it at once contracts on one side, so 

 as to form a curve over the surface of the body, and to embrace it close- 

 ly, till seven or eight coils have been formed around it, and this is done 

 so rapidly, that three turns of the helix are sometimes made in fifteen 

 minutes. The nature of the body presented has no influence on the 

 process, the tendrils coiling as rapidly over one substance as another. 

 As these and other phenomena cannot be accounted for by any action 

 so slow as the ordinary process of nutrition, it seems necessary to admit 

 the existence of irritability as a vital property inherent in the tissues of 

 the tendril ; this property is found to cease when the tendril is separated 

 from the parent, and, like the irritability of sensitive plants, it is excited, 

 modified, and even suspended or destroyed, by the influence of vegetable 

 or mineral poisons. 



The next subject examined is the inclination of stems towards the 

 light, which De Candolle ascribed to the more rapid and more complete 

 solidification of the tissue by exhalation and fixation of carbon on the 

 side of the stem exposed to the light. Prof. Macaire first inquires, 

 if such a special attraction is exercised by light on the green parts of 

 a plant as to cause the entire plant to move towards light, if permitted 

 to do so; and his experiments on duck-weed, and on germinating 

 plants of various kinds, attached to cork floats, lead him to a negative 



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