. A textbook of botany for colleges and universities ... Botany. 55° ECOLOGY chies, one stem circuit making a complete round of the spiral; fig. 729). Progres- sively more complicated arrangements are \, |, f, ^, the latter meaning, for example, that there are thirteen orthostichies, and that five stem circuits are necessary for a complete round of the spiral, the fourteenth leaf being above the first, etc. Spiral phyllotaxy is advantageous from the standpoint of leaf lighting, since it results in relative remoteness between the members of the same orthostichy; the screw pine {Pandanus) gives
. A textbook of botany for colleges and universities ... Botany. 55° ECOLOGY chies, one stem circuit making a complete round of the spiral; fig. 729). Progres- sively more complicated arrangements are \, |, f, ^, the latter meaning, for example, that there are thirteen orthostichies, and that five stem circuits are necessary for a complete round of the spiral, the fourteenth leaf being above the first, etc. Spiral phyllotaxy is advantageous from the standpoint of leaf lighting, since it results in relative remoteness between the members of the same orthostichy; the screw pine {Pandanus) gives an admirable illustration of such arrangement, due to a high-ranked spiral. A relation sometimes is claimed to exist between phyllotaxy and leaf size, that is, complex spirals are supposed to be associated with small leaves and simple spirals with large leaves. Small leaves often occur in many ranks (as in Yucca, Lycopoditim, and Polyirichum, figs. 901, 265), and large leaves likewise; probably such relations are fortuitous. The causes of variations in phyllotaxy are not definitely known. A common theory has been that leaf position is due to mechan- ical influences exerted in the bud, such as the pressure of older parts upon those just developing; in recent years, however, many serious objections to this view have been advanced. The \ system that commonly obtains in Phyllocactus is changed to a J system when the plant is grown in the dark; similar changes have been ob-. FlG. 789.—A mesophytic forest with a luxuriant undergrowth of ferns (Osmanda), whose compound leaves jwrmit the sifting of light and the consequent illumination of subjacent foliage; Porter, Indiana. —Photograph supplied by Meyers. served in Lycopodium and in Schistostega (figs. 787, 788), and in Caulerpa, " leaves" occur only on the lighted side. In any case, no connection need be sought between the causes and the advantages of the various kinds of phyllotaxy. Compound and small leaves. — Divid
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
