454 TRANSFORMATION OF ENERGY 



but in the full-grown shoot the leaves are no longer so, but all are arranged 

 in two rows along the sides of the branch. This alteration in position is due to 

 torsion of the internodes, and since also it takes place in darkness (SCHWENDENER 

 and KRABBE, 1892) it must be dependent on gravity, although in the majority 

 of cases such torsions are due to light (Lecture XXXVI). 



Since the mechanics of all these torsions have not as yet been fully explained 

 we will not consider them further. Leaves, however, suggest another aspect 

 from which we may study geotropism. Hitherto we have referred all geotropic 

 movements to growth, and have emphasized the fact that they take place only 

 if the organ in question is still in a state of growth or is capable of starting 

 growth anew. In the leaves of many plants (e.g. Leguminosae and Oxalidaceae) 

 movements occur which may take place without any growth and merely by 

 inequality of osmotic pressure on opposite sides of the petiole. 



Leaves and petioles do not possess the power throughout their entire 

 extent of altering their state of turgor and so elongating or shortening opposite 

 sides ; on the contrary this peculiarity is limited to one special organ, which may 

 be identified externally. Since these organs occur at the base of the leaf -stalk, 

 or (as in pinnate or bipinnate, &c., leaves) at the bases of the pinnae as well, 

 forming relatively short connecting regions between non-motile parts, we may 

 term them articulations. For the most part they stand out from parts in the 

 vicinity by reason of their greater thickness, and hence are termed ' cushions ' 

 or pulvini. Their anatomical structure is very characteristic and closely 

 related to their function. An examination of the pulvinus of Phaseolus multi- 

 fiorus shows that all the vascular bundles run together in a single axial stand 

 imbedded in parenchyma. The walls of the parenchyma cells are very elastic 

 and extensible, and when in a state of turgor marked differential tissue tensions 

 are induced with the scarcely extensible vascular bundle. If the parenchyma 

 be isolated it elongates very considerably, while the vascular system contracts 

 scarcely at all. As may be easily seen, an increase in turgescence on one 

 side or a decrease on the other, or the occurrence of both phenomena simul- 

 taneously, must result in an elongation of one side and contraction of the 

 other, so that the articulation bends. The vascular bundle is of course also 

 bent but undergoes no change in length. Bending of the joint naturally results 

 in a passive movement of the parts of the leaf distal to the articulation. 



Movements in the pulvini may be induced by the most varied external 

 stimuli, as also when the leaf is turned upside down by inversion of the stem 

 (SACHS, 1865). In order to bring it back once more to the normal position, for 

 the most part not only are simple curvatures necessary but torsions also 

 occur, which have been even less investigated here than in other cases. In 

 the curving of the joint there is no permanent elongation of the convex side, 

 as PFEFFER (1875) demonstrated by microscopic measurements, and after the 

 inversion of the stem the leaf soon resumes its previous position. From the 

 fact that no growth can be determined, it may be concluded that the curving 

 here is due to an increase of osmotic pressure on the convex side and a corre- 

 sponding diminution on the concave, a conclusion confirmed by the plasmolytic 

 method. Thus HILBURG (1881) found that in the articulation of Phaseolus 

 plasmolysis began to appear on the morphological upper side after a geotropic 

 stimulus, only when a 4 per cent, solution of potassium nitrate was used, 

 although before the stimulus a 3 per cent., solution was sufficient ; conversely, 

 the osmotic value of the cell contents of the under side, as measured in terms 

 of potassium nitrate, fell from 3^ per cent, to 3 per cent. When the stimulus 

 was permitted to affect the organ for a longer period growth, it is true, finally 

 ensued on the convex side, and when the stem was placed once more in its 

 normal position the leaf was incapable of attaining its original orientation. 

 Although geotropic curvature induced by turgor is limited to the articulations 

 of certain leaves, it is still theoretically of great interest, since it is probable 



