CURVATURES OF SHOOT- AXES DURING GEOTROPIC ERECTION. 687 



It may thus be said, in a completely erect shoot the strongest and finally per- 

 manent curvature is at that place which offers most resistance to the influence of 

 geotropism. 



The erection of a shoot-axis by geotropism is, as we see, the result of somewhat 

 complicated movements, which are influenced by numerous accessory causes ; 

 nevertheless the final erection in the case of fairly sensitive shoot-axes is mathe- 

 matically exact, i. e. even the feeblest curvatures which arise in the course of 

 the geotropic movement become at last so compensated that the growing parts again 

 become perfectly straight and upright. This is seen with extraordinary clearness, for 

 example, in the case of one of our most remarkable water-plants, Ulricularia vul- 

 garis ; the primary shoot of this plant, with its finely divided leaves, floats horizontal 

 and quite free on the water, only the flower-scape rising perfectly vertically, 15-20 cm, 

 into the air, although the slightest obliquity suffices to turn the horizontally 

 floating main shoot round, so that the flower-scape falls horizontally on the water. 



It scarcely needs mention that what has been so fi\r said as to the upward 

 curving refers to the commonest cases only ; there are several others in addition, 

 where the behaviour is different on account of the organisation of the parts concerned. 

 I will only dwell upon two cases, in both of which, however, it is not strictly 

 the geotropic erection of shoot-axes which is concerned, but of peculiarly organised 

 parts of leaves. I here refer to the so-called nodes on the haulms of Grasses, and 

 the motile organs of compound leaves, especially those of the Leguminosae, already 

 so fully considered. These organs, when their leaves are completely developed, 

 also cease to grow, it is true, but their anatomical and physiological constitution 

 betray the fact that they are, so to speak, in a persistent condition of youth ; their 

 parenchyma is highly turgescent, and the lignification of the bundles is suppressed, 

 and, above all, these organs (especially the nodes of Grasses) can be impelled to renew 

 their growth simply by being laid in a horizontal or oblique position. 



On the fully developed haulms of the Grasses, to which group our cereals belong 

 — Wheat, Barley, Rye, Oats, ]\Iaize, &c. — there are to be noticed at considerable 

 distances (5-30 cm. or more) apart certain knot-like swellings, sharply marked off 

 from the thin cylindrical parts, and usually coloured differently. If the haulm is 

 split longitudinally, it is easily seen that the knot in question is nothing further 

 than the annular and strongly thickened basal zone of a leaf-sheath, which envelopes 

 the internode of the shoot -axis above the knot as a very thin but stiff inrolled 

 sheath. In young haulms these internodes are very tender and flexible ; the rigidity - 

 and elasticity of haulms not yet quite mature depend entirely upon the firmness 

 of these leaf-sheaths. 



On bending a haulm sharply above the soil, so that the whole of its apical 

 portion, about i m. long, comes to lie horizontally, it is noticed after 2-4 days that 

 knee-like curvatures have been formed at its nodes, in consequence of which the 

 apical portion of the haulm has again erected itself vertically; as a rule 2-3 nodes 

 take part in this change. It is sufficient moreover if pieces of the haulm con- 

 taining one or a few nodes are cut off and placed horizontally with one end, it matters 

 not which, in damp sand, to obtain after a few days the same strong knee-like 

 curvatures. These knee -like formations are produced exclusively by the nodes 

 referred to, the other portions of the haulm exhibiting no geotropic curvatures what- 



