The principles of biology . ch it occurs; and tracing it wiH prepare us for under-standing how, and when, metamorphoses still greater in de-gree, take place. § 188. If we examine a branch of the common bramble,when in flower or afterwards, we shall not imfrequently finda simple or undivided leaf, at the insertion of one of thelateral flower-bearing axes, composing the terminal cluster offlowers. Sometimes this leaf is partially lobed; sometimescleft into three small leaflets. Lower down on the shoot, ifit be a lateral one, occur larger leaves, composed of threeleaflets; and in some of these, t
The principles of biology . ch it occurs; and tracing it wiH prepare us for under-standing how, and when, metamorphoses still greater in de-gree, take place. § 188. If we examine a branch of the common bramble,when in flower or afterwards, we shall not imfrequently finda simple or undivided leaf, at the insertion of one of thelateral flower-bearing axes, composing the terminal cluster offlowers. Sometimes this leaf is partially lobed; sometimescleft into three small leaflets. Lower down on the shoot, ifit be a lateral one, occur larger leaves, composed of threeleaflets; and in some of these, two of the leaflets may belobed more or less deeply. On the main stem, the leaves,usually still larger, will be foimd to have five leaflets. Sup- THE MORPHOLOGICAL COMPOSITION OF PLANTS. 29 posing the plant to be a well-grown one, it will furnish altgradations between the simple, very small leaf, and the largecomposite leaf, containing sometimes even seven 50 to 64, represent leading stages of the What determines this transition ? Observation shows that thequintuple leaves occur where the materials for growth aresupplied in greatest abundance; that the leaves become less 30 MORPHOLOGICAL DEVELOPMENT. and less compound, in proportion to tlieir remoteness from themain currents of sap; and that where an entire absence ofdivisions or lobes is observed, it is on leaves within theflower-bunch: at the [place, that is, where the forces thatcause growth are nearly equilibrated by the forces thatoppose growth; and where, as a consequence, gamogenesis isabout to set in (§ 78). Additional evidence that the degreeof nutrition determines the degree of composition of the leaf,is furnished by the relative sizes of the leaves. Not only, onthe average, is the quintuple leaf much larger in its total areathan the triple leaf; but the component leaflets of the one, areusually much larger than thoseof the other. The like con-trasts are still more marked between triple le
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectbiology, bookyear1864
