210 



SCIENCE 



[N. S. Vol. XLIV. No. 1128 



SPECIAL ARTICLES 



ON THE ASSOCIATION AND POSSIBLE IDEN- 

 TITY OF ROOT-FORMING AND GEOTROPIC 

 SUBSTANCES OR HORMONES IN 

 BRYOPHYLLUM CALYCINUM 



Recent experiments have led me to results 

 which suggest that the substances responsible 

 for root-formation in the stem of Bryophyllum 

 calycinum are associated or possibly identical 

 with the substances responsible for geotropic 

 curvatures of the stem of this plant. 



la. When we cut out a piece of the stem of 

 Bryophyllum and suspend it horizontally in a 

 vessel saturated with water vapor, the stem 

 will bend to such an extent that it assumes 

 the shape of a U, the concave side being on 

 the upper side. It was found that this geo- 

 tropic curvature is due to a growth (or some 

 other form of active stretching) of the cortex 

 in the convex region of the lower half of the 

 stem. The upper half of the stem is bent pas- 

 sively through the growth of the lower half. 

 This was ascertained by measurements on 

 marked stems split longitudinally, and sus- 

 pended horizontally. 



16. It was found that root-formation appears 

 generally in that node around which the curva- 

 ture takes place and that it is confined in the 

 bending region to the nodes on the lower side 

 of a horizontally suspended stem. It is thus 

 seen that the geotropic growth (or active 

 stretching) and the root-formation both take 

 place on the lower side and in the same region 

 of the stem. 



2a. When we cut out a piece of stem from 

 Bryophyllum containing from four to seven 

 nodes (but with the two most apical nodes cut 

 off) and if we remove all the leaves such a 

 stem will form roots at the two most basal 

 nodes (and sometimes also at the basal surface) 

 and new shoots at the two most apical nodes, 

 but this new growth is extremely slow. If, 

 however, a leaf is left on the stem the new 

 organs will grow out much more rapidly. 



26. When such a stem without leaves is sus- 

 pended horizontally it will bend geotropically, 

 but the bending will take place very slowly. If, 

 however, a leaf is left on the stem the geotropic 

 curvature takes place with much greater 

 rapidity. 



3a. When we remove all but one apical leaf 

 on the lower side of such a horizontally sus- 

 pended stem, the stem will form roots first in 

 the second node from the leaf; but only from 

 the node on the under side of the stem. Roots 

 will also grow out from the two most basal 

 nodes. 



36. In the same stem the geotropic curva- 

 ture will occur in the region where the first 

 growth of roots takes place; namely around 

 the second node behind the leaf. 



4a. When all the leaves are removed with 

 the exception of one leaf in the basal node (on 

 the under side of the horizontally suspended 

 stem), root-formation will be scant and will 

 only take place at the cut surface at the basal 

 end of the stem behind the leaf; and some- 

 times also from the axilla of the leaf. 



46. In such a stem the geotropic curvature 

 is generally considerably less than when an 

 apical leaf is left and is confined to the piece 

 of internode behind the leaf and to the imme- 

 diate neighborhood in front of the leaf. 



5. In all these experiments the region of 

 curvature (and of growth of the cortex) coin- 

 cides with the region where the most rapid 

 growth of the roots takes place (or where root- 

 forming substances or hormones collect). 



6. The effect of the position of a single leaf 

 on the stem is much more striking when we 

 remove the upper half of the cortex in a hori- 

 zontally suspended stem of Bryophyllum. 

 Such stems become at once very strongly con- 

 vex on the upper side, due to the release of the 

 passively contracted wood and pith on the 

 upper side, where the cortex is removed. When 

 in such a stem all the leaves are removed ex- 

 cept the one on the lower side at the apical 

 end of the stem, the latter will gradually over- 

 come the convexity on the upper side and as- 

 sume the geotropic U shape with the concavity 

 on the upper side due to geotropic growth of 

 the cortex on the lower side of the stem, in the 

 region around the second node behind the leaf. 

 If, however, the leaf is left at the basal end no 

 geotropic curvature will occur (at least none 

 appeared as long as the stems were observed). 

 If the cortex is removed on the lower side no 

 geotropic curvature is possible since this curva- 



