TORSION. 
^59 
smaller and nearer to the apex. If mid-ribs of leaves are separated in this last 
stage of growth and fixed upright in a damp and dark place (<?. g. in wet sand in a 
spacious closed zinc box), they will continue to grow for some time ; and since growth is 
more vigorous on the inner (anterior or upper) side, they will curve concavely on the 
posterior (or under) side, the curvature being however partially counteracted by geo- 
tropism. If separated mid-ribs of leaves are fixed horizontally in wet sand, so that the 
median plane lies horizontal, the epinastic curvature will take place without hindrance 
in a horizontal direction ; but a -geotropic curvature will at the same time ensue in a 
vertical plane, so that the two kinds combine to produce an obliquely ascending position. 
If, on the other hand, two similar mid-ribs are separated and fixed horizontally in wet 
sand, with the posterior side in one case below, in the other case above, geotropism will 
act in the former in opposition to epinasty, while in the latter the two will cooperate ; 
and the consequence will be that in the former case the epinastic curvature will be more 
or less neutralised, while in the latter a strong curvature will take place upwards, the two 
forces acting in unison. 
Phenomena of the same kind are produced by a combination of epinasty with helio- 
tropism, if the separated mid-rib is fixed vertically in wet sand in a closed vessel into 
which light is admitted from one side through a glass plate. Heliotropism is generally 
but not always exhibited, and is then always positive ; but in all the cases hitherto ob- 
served is too weak to overcome epinasty. It will be seen from what we have said that 
all these movements of the mid-rib will be much less considerable when it is still in 
connexion with the lamina. Petioles show in general the same phenomena as mid- 
ribs, but their motions which result from heliotropism, geotropism, and epinasty are 
unimpeded. 
(b) Bilateral secondary shoots, such as branches of an inflorescence, horizontal or erect 
leafy branches, and stolons, were experimented on in a similar manner. It was thus proved 
that the branches of the inflorescence of Isatis tinctoria, Archangelica offic'malis, Crambe 
cordifolia, and all others that have been observed, the horizontal branches of Pyrus Mai us ^ 
Asperugo procumbens, &c., as well as the runners of Fragaria, Potentilla reptans, Ajuga 
reptans, &c., are epinastic. When fixed horizontally in wet sand, they all curl upwards, 
whether the side that normally faces downwards (the posterior side) be placed below 
or above, but in the latter case more strongly, because geotropism and epinasty then 
cooperate. In some species (as Tilia and Philadelphus) a branch, when stripped of 
leaves and placed in its normal position, did not curl upwards, while one placed in a 
reverse position did so, proving that there was in these cases an equilibrium between 
geotropism and epinasty. The horizontal branches of Prunus A-vium, U/mus campestris, 
Cory/us A'vel/ana, and some other plants were found on the other hand to be hyponastic ; 
when laid horizontally in their natural position they curved upwards, but downwards 
if reversed, because their hyponasty was stronger than their geotropism. 
Similar experiments to those made on petioles with respect to heliotropism 
showed in many cases the absence of this phenomenon, especially in the case of stolons ; 
' and that in other cases it was always positive, but too feeble to overcome the influence 
of their epinasty. In the case of branches, especially such as are long and slender, 
more account must be taken of weight in modifying the direction of growth than in 
that of leaves. The removal of the leaves {e.g, in Corylus) is in this case followed by a 
sudden curving upward, the result of elasticity; but this is subsequently intensified by 
geotropism and in many cases (as in Abies) also by hyponasty. 
It may be left to the ingenuity of the student to ascertain the conditions determining 
the direction of an organ in any particular case, from the points of view stated above. 
Sect. 23. — Torsion \ Organs of any considerable length very commonly ex- 
^ H. de Vries in the second Heft of the Proceedings of the Würzburg Botanic Institute, 1871, 
p. 272. — Wichura in Flora, 1852, No. 3, and Jahrbuch lür wissensch. Bot. vol. II, i860. — Braun in 
Bot. Zeitg. 1870, p. 158. 
