File:Elementary botany (1905) (14783089582).jpg

Identifier: elementarybotan00atki (find matches)
Title: Elementary botany
Year: 1905 (1900s)
Authors: Atkinson, George Francis, 1854-1918
Subjects: Botany
Publisher: New York : Holt
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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ny directions. Occasionally one finds a divided or compound leaf in such aposition that one portion, because of being shaded above, receivesthe stronger fight stimulus from the side, while the other portionis lighted from above. If this relation continues throughoutthe growth-period of the leaf the leaflets of one portion may liein a different plane from those of the other portion. In suchcases, some of the leaflets are permanently twisted to bring theminto their proper light relation. 404 RELATION TO ENVIRONMENT. V. Leaf Patterns. MOSAICS, OR CLOSE PATTERNS. 774. Where the leaves of a plant, or a portion of a plant, areapproximate and arranged in the form of a pattern, the leavesfitting together to form a more or less even and continuous sur-face, such patterns are sometimes termed mosaics, since therelation of leaves to one another is roughly like the relation ofthe pieces of a mosaic. A good illustration of a mosaic is pre-sented by a greenhouse plant Fittonia (fig. 441). The stems
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Fig. 441.Fittonia showing leaves arranged to form compact mosaic. The netted vena-tion of the leaf is very distinctly shown in this plant. (Photo by the Author.) are prostrate and the erect branches quite short, but it mayhave quite a wide system by the spreading of the runners; thebranches of such a length that the leaves borne near the tips allfit together forming a broad surface of leaves so closely fittedtogether often that thestems cannot be seen. The advantageof a mosaic over a separate disposition of leaves at somewhatdifferent levels is that the leaves do not shade one another. Wereall the light rays coming down at right angles to the leaves, therewould not be any shading of the lower ones, but the obliquerays of light would be cut off from many of the leaves. In thecase of a mosaic all the rays of light play upon all the leaves.Some of the mosaics which can be observed are as follows: FOLIAGE LEAVES. 405

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