. Practical text-book of plant physiology. Plant physiology. 84 RELATION OF PLANTS TO GRAVITATION ing while those on the upper are convergent. Examine for dif- ferences in structure. 128. The Geotropic Relations of Dorsiventral Organs. The greater number of dorsiventral organs, such as leaves, are dia- geotropic, although many well- known exceptions occur. Such organs are in equilibrium only when the axis subtends a certain fixed angle with the line of grav- ity, and when the two sides oc- 'a~ b cupy their proper and relative po- Fig. 38. A, stem and branches of sitions. The geotropic reaction
. Practical text-book of plant physiology. Plant physiology. 84 RELATION OF PLANTS TO GRAVITATION ing while those on the upper are convergent. Examine for dif- ferences in structure. 128. The Geotropic Relations of Dorsiventral Organs. The greater number of dorsiventral organs, such as leaves, are dia- geotropic, although many well- known exceptions occur. Such organs are in equilibrium only when the axis subtends a certain fixed angle with the line of grav- ity, and when the two sides oc- 'a~ b cupy their proper and relative po- Fig. 38. A, stem and branches of sitions. The geotropic reactions of a pine tree meter, in height from such organsmay show tw0 move. which the top has been removed. B, same * year later showing upward ments: a curvature to bring the curvature of a branch seven years old. axis toward the horizontal, and a terJost- twisting or torsion to place the surfaces in their normal relative positions. These reactions are generally complicated with phototropic movements in plants growing in the open, and are not capable of easy analysis. 129. Rotation and Curvature of Petioles of Dorsiventral Leaves in Response to Geotropic Stimuli. Secure a vigorous specimen of any species of Helianthus growing in a pot. Place the axis of the plant in a horizontal position in a dark room and note the positions of the leaves with respect to the stem, and ascertain what curvatures and torsions have taken place. 130. Geotropic Curvatures in Organs in which Growth in length, has Ceased. It has been a mattef of common observation that the branches of many trees, two, three or more years old, undergo upward curvatures. Such curvatures have been observed in branches of many conifers cm. in thickness and progeotropic curvatures in the weeping branches of many trees have also been seen. Jost records the apogeotropic curvature of a branch of a fir seven years old1 (Fig. 38). •Vochting, H. Organbildung im Pflanzenreich. 2 185. 1884. Jost, L. Ueber einige Eigenthiimlichk
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