File:Plant life, considered with special references to form and function (1898) (20715141946).jpg

Title: Plant life, considered with special references to form and function
Identifier: cu31924001307846 (find matches)
Year: 1898 (1890s)
Authors: Barnes, Charles Reid, 1858-1910
Subjects: Plant physiology
Publisher: New York, H. Holt & company
Contributing Library: Cornell University Library
Digitizing Sponsor: MSN

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THE MOVEMENTS OF ELAXTS. 195 Light thus jjlays an important part in determining the position of organs. As a rule radial organs are cither positively heli- otropic, as the stems and leaf-stalks, or negati\el)' heliotropic, as the roots. Dorsiventral organs, such as leaves, are all transversely heliotropic, assuming a position at right angles to the incident rays, which is the most favorable position possible for the manufacture of food by the green parts (fig. 190). Intense light, ho\ve\-er, may bring about a different reaction, so that the leaves set themselves edgewise to the
Text Appearing After Image:
Fig. iQr.—Leaf mosaic formed by a horizontal shoot of Norway maple. The lengthen- ing of the petioles of individual leaves to avoid shading of the blade is marked. About one-third natural size.—After Kemer. direction of the rays. A fixed light position is usually reached by leaves by the time they become mature, and this is generally at right angles to the source of greatest light. Branches of trees show the leaves so arranged as to size and position that they shade each other as little as possible, form- ing the so-called leaf mosaics (figs. 191 to 193I. The leaves of window plants also exhibit these movements very strikingly,

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