646 REPORT— 1889. 



growth of the leaves, and that since the leaves are inherently epinastic, their 

 renewed growth under the influence of light is necessarily epinastic also. 



The author then adduces a numher of observations made on various plants, 

 such as species of Helianthus, Fiichsia, Dahlia, Impatiens, which prove that 

 epinasty is spontaneous, since the leaves of these plants showed well-marked 

 epinastic curvature when kept in darkness for twenty-four hours or more. 



With regard to the second point, the assumption that dorsiventral members 

 are negatively geotropic is based chiefly on the behaviour of young radical leaves 

 of various plants such as Plantago, Taraxacum, Sec, and of the thallus of Mar- 

 chantia, when kept in darkness in the normal position. The author cites experi- 

 ments in which the plants were rotated on the clinostat in darkness — the influence 

 of gravitation being therefore eliminated^ — the result of which was that the leaves 

 performed exactly the same movements under these conditions as when the plants 

 were standing in the normal position. From this it follows that the movement 

 in question cannot be due to the action of gravitation, but must be ascribed 

 to inherent causes determining the more rapid growth of the under side of the 

 leaf, — that is, to hyponasty. The same result was obtained in experiments with 

 Marchantia. 



The author then goes on to analyse the causes which determine the position 

 ultimately assumed by dorsiventral members when growing under normal con- 

 ditions. These causes he believes to be three : the inherent mode of growth of the 

 member ; the action of light ; the action of gravitation. 



The author considers that the action of light is such as to induce the member 

 to place its upper surface at right angles to the incident rays ; he agrees, therefore, 

 with Frank, C. Darwin (' Movements of Plants,' 1880), and F. Darwin (' Journ. 

 Linn. Soc.,' xviii., 1881) that dorsiventral members are diaheliotropic. The fact 

 on which he bases this conclusion is this, that leaves which have become curved^ 

 either epinastically or hyponastically, in consequence of having been kept in dark- 

 ness, return to the horizontal position when fully exposed to light. In these cases 

 light produces two opposite eSects : it causes epinastically curved leaves to rise 

 into the horizontal plane, and hyponastically curved leaves to sink down into the 

 horizontal plane. It seems impossible to explain these facts otherwise than on the 

 assumption that epinastic and hyponastic members possess the same kind of 

 irritability with reference to the directive action of light — an irritability which 

 causes them both to assume the same position under its influence. 



With regard to the action of gravitation, the author, having proved that 

 hyponastic dorsiventral members are not negatively geotropic, suggests that this 

 is probably true of all kinds of dorsiventral members, since there is absolutely no 

 evidence to prove the negative geotropism of epinastic members. This being so, 

 the question arises whether or not dorsiventral members are at all sensitive to the 

 action of gravitation, and, if so, what the nature of their reaction may be. The 

 author does not profess to have fully investigated this subject, but he describes 

 some observations made on plants placed in darkness, with their long axes hori- 

 zontal, which tend to prove that the action of gravitation upon the leaves is to 

 bring them into such a position that their upper surfaces are horizontal and face 

 the zenith. He therefore supports Frank's theory of diageotropism. In these 

 observations the efifect of epinasty and hyponasty had to be taken into account in 

 ascertaining as far as possible the nature and the extent of the efiect due to 

 gravitation. 



These conclusions will be rendered more intelligible when they are applied to 

 the elucidation of particular cases. For instance, the leaves of HelianthuB, as 

 mentioned above, are strongly epinastic — so much so that in darkness the leaves 

 curve downwards in opposition to diageotropism ; when fully exposed to light the 

 leaves are horizontal, the combined influence of diaheUotropism and diageotropism 

 being sufficiently strong to counteract epinasty. Similarly, the young leaves of 

 Plantago have been shown to be hyponastic — so much so that in darkness their 

 hyponasty overcomes their diageotropism ; the horizontal position assumed in 

 bright light is the result of the co-operation of diaheliotropism and diageotropism 

 in opposition to hyponasty. 



