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XLII. On Divergence as the Cause of Motion in Plants. By 

 Henry Johnson, M.D.* 



IN a former communication I have given a description of 

 -* the phaenomena of divergence, and I have endeavoured to 

 prove the analogy of this property to the irritability of the 

 animal system f. I shall now attempt to show that the cause 

 of divergence is the cause of the motion in plants. 



I do not think it necessary to enumerate the many instances 

 and varieties of vital or other motion which are met with in 

 works on vegetable physiology. Instead of extending my re- 

 marks to vegetable motion in general, I shall at present con- 

 fine my observations to one kind, which, from its frequent oc- 

 currence and the very obvious nature of its effects, is parti- 

 cularly well suited to experimental illustration. 



It seems to be essential to the healthy condition of most 

 plants, that the upper surface of their leaves or their blossoms 

 should be exposed to the light of the sun. I think, indeed, that 

 there are very few, if any, of the more perfect vegetables, to 

 which it is immaterial whether their leaves and flowers be 

 exposed to the light or not. Such plants have, therefore, 

 been endowed with the power of directing these parts to the 

 light, and this motion is effected by a curvature of the stem, 

 leaf-stalk, or flower-stalk. If a young sunflower or hollyhock 

 be laid prostrate, the stem curves, and the apex becoming 

 erect, the leaves are again placed in their usual relation to 

 the rays of light. Where the stem is rigid or fixed, it is the 

 leaf-stalks which perform this motion, as in ivy, of which 

 I have seen the leaf-stalks contorted in the most extraordinary 

 manner from their efforts to bring the glossy surface of the 

 leaves under the influence of the light. I remember to have 

 observed a Campanula which had been blown down by the 

 wind, and the stem of which being too rigid and ligneous for 

 it to be capable of curvature, the flower-stalks became curved, 

 so as to erect the fruit, present it to the light, and preserve it 

 from the humidity of the soil. 



That motion of vegetables by which they regain the erect 

 posture when laid prostrate, or direct their leaves towards 

 the light, (with the exceptions afterwards to be mentioned,) is 

 always attended with curvature of the stem, leaf-stalk, or 

 flower- stalk ; and I proceed to state the experiments and ob- 

 servations which convince me that the property of diver- 

 gence is the essential cause of this curvature and the conse- 

 quent motion above described. 



* Communicated by the Author. 



f See Lond. and Edinb. Phil. Mag., March 1835, vol. vi. p. 164. 



