UNEQUAL GROWTH. 769 



sensitive to the influence of light, and the upper side to that of gravitation than 

 the reverse. But this hypothesis is superfluous if it is borne in mind that in this 

 case epinasty works concurrently with heliotropism and geotropism, and hence 

 much stronger curvatures must take place than in the normal position where the 

 former acts in opposition to the two latter forces. 



The results here described are derived from the experiments of de Vries, which have 

 been already quoted. For the following I am also indebted to him. 



(a) Leaves. If a strongly developed mid-rib is separated from a leaf in active growth, 

 it curls up concavely on the under side, showing that a tension exists between it and 

 the mesophyll. De Vries found this to be the case in nearly two hundred species, with 

 only a few exceptions. This curvature does not take place equally strongly at all ages; 

 in leaves which have but just emerged from the bud it does not occur at all ; it increases 

 with age, and attains its maximum when the leaf is nearly fully grown, then again 

 decreases, and altogether disappears when the leaf has reached full maturity. This 

 tendency to curve is at first apparent along the whole length of the mid-rib ; it dis- 

 appears first of all at the base, the part capable of curvature becoming constantly 

 smaller and smaller towards the apex. If mid-ribs of leaves are separated in this last 

 stage of growth and fixed upright in a damp and dark place {e.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 suspended 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 placed horizontally in wet 

 sand, with the posterior side in one case above, in the other case below, 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 placed 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 mesophyll. Petioles show in general the same phenomena as 

 mid-ribs, but their m.otions which result from heliotropism, geotropism, and epinasty 

 are unimpeded. 



(b) Bilateral secondary shoots, such as branches of an inflorescence, horizontal or erect 

 leaves, and stolons, would exhibit precisely similar phenomena. It may be proved also 

 that the branches of the inflorescence of Isatis tinctoria, Archangelica officinalis, Crambe 

 cordifoUa, and all others that have been observed, the horizontal branches of Pyrus Malusy 

 Asperugo procumbens, &c., as well as the runners of Fragaria, Potentilla reptans, Ajuga 

 reptans, &c., are epinastic. When placed horizontally in wet sand, they all curl upwards, 

 whether the side that normally faces downwards (the posterior side) was 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 auium, JJlmus campestris, 



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