GEOTROPISM. H 



453 



morphic forms, as do also leaf-blades, which in most cases are markedly dorsi- 

 ventral. As an example of a dorsiventral flower which owes its position to 

 the influence of gravity only, and not, as in many other cases, to light, we 

 may take Aconitum napellus, which has been studied by NOLL (1885-7). If 

 the inflorescence of this plant be so bent that the terminal portion of the axis 

 with its buds points vertically downwards, and if it be fixed in this position 

 in some appropriate way, after a short time the peduncle becomes curved as 

 shown in Fig. 141. The curvature ceases when the upper part of the peduncle 

 comes to form once more an angle of 30-50 with the direction of gravity, that 

 is, when the hooded sepal has again attained the upper position. If the lie 

 of the flower be regulated by the force of gravity only, this lie (Fig. 141, //) must 

 be that of rest. The relation of the flower to the axis of inflorescence has also 

 to be considered, for it is only when the opening of the flower points outwards 

 that insects can visit it and the flower can perform its normal functions. 

 Thus we see that, after this median bending inwards of the peduncle, there 

 follows a complex movement resulting in a torsion of the peduncle and an 

 outward turning of the flower itself. We must leave it undetermined whether, 

 as is to be expected from NOLL'S account, a purely geotropic movement is always 

 combined with another autonomous one induced by internal causes (NOLL'S exo- 

 tropism), or whether (SCHWENDENER and KRABBE, 1892) a torsion may take 

 place without any median inbending. This 

 much at least is certain that in these orienta- 

 ting movements in flowers of this kind a 

 correlative influence of the axis plays a part ; 

 this is especially noticeable in the flowers of 

 the Orchidaceae. The flowers of this order 

 are orientated inversely in the bud, i.e. the 

 labellum is posterior, but owing to a torsion 

 in the ovary during the evolution of the bud 

 they come to assume the normal position. 

 It is possible to prevent this torsion, how- 

 ever, and compel the flower to open in the 

 inverted position if the plant be rotated on 

 a klinostat or if the inflorescence be fixed in 

 the inverted position. The torsion must, 

 therefore, be induced by gravity. If, how- 

 ever the axis be cut off above a flower which has not yet suffered torsion, 

 the flower assumes its normal position by a simple curvature without any 

 torsion, and inclines itself outwards over the top of the cut end of the axis ; 

 that is, it performs only the first of the movements which have already been 

 described as occurring in Aconitum. 



Foliage leaves, as we might expect, behave in principle in the same way 

 as do dorsiventral flowers. If the axis be fastened in the inverse position they 

 could regain their geotropic angle and correct orientation of upper and under 

 sides by a simple geotropic axial curvature of the petiole or base of the lamina ; 

 they would, however, succeed in carrying out their orientating movements by 

 turning the tips of their blades inwards, though they seldom find space enough 

 available. As a matter of fact, we find them performing the same movements 

 which we have found characteristic of flowers (NoLL, 1887), i.e. a median inward 

 curvature is often followed, as in Aconitum, by an exotropic movement, while 

 in other cases, especially leaves with short petioles, the same result is achieved by 

 torsion only. It is impossible to decide at present whether this torsion consists, 

 as NOLL believes, always of two combined movements, or whether it is a single 

 movement (geo-torsion, SCHWENDENER and KRABBE, 1892). 



Other torsions also occur, induced by gravity, as in many plagiotropic 

 shoots, e.g. Philadelphia. The leaves in the bud of this plant are decussate 



I'tg. 141. I 



napellus in tw 



Inverted flower of Aconitum 

 o stages. After NOLL ( 1885-7). 



