. Text-book of botany, morphological and physiological. Botany. RELATIVE POSITIONS OF LATERAL MEMBERS. 193 -cases of decidedly bilateral construction the genetic spiral might be imagined just as well, and with the same divergence^ ascending right or left, by which of course it loses its importance for any morphological conclusion, as much as if one supposed it to change its direction from leaf to leaf. It is principally in upright axes with solitary leaves arranged in three, four, five, or more directions, that the spiral construction appears conformable to nature, and agrees with the symmetri
. Text-book of botany, morphological and physiological. Botany. RELATIVE POSITIONS OF LATERAL MEMBERS. 193 -cases of decidedly bilateral construction the genetic spiral might be imagined just as well, and with the same divergence^ ascending right or left, by which of course it loses its importance for any morphological conclusion, as much as if one supposed it to change its direction from leaf to leaf. It is principally in upright axes with solitary leaves arranged in three, four, five, or more directions, that the spiral construction appears conformable to nature, and agrees with the symmetrical relationships of plants, of which more will be said hereafter. The spiral construction proves to be opposed to nature in bilateral structures, especially in creeping or climbing stems, and in lateral branches. In those cases in which the spiral construction may be employed naturally to elucidate the relative positions of the members, two cases may be distinguished, according as the divergences, on the one hand, are very unequal and change abruptly, or, on the other hand, are nearly or quite equal to one another or only change gradually. In the first case the members appear to be arranged irregularly and without order, as the foliage-leaves on the stem of Fritillaria imperialis (Fig. 151), the flowers on the rachis of the raceme of Triglochin palustre or of many Dicotyledons. When the change of divergence on the same axis is abrupt, it may also' ap- pear more natural to represent the phyllotaxis by two homodromal spirals instead of one, as in many species of aloe, where the shoots commence with leaves arranged in two rows, and then pass over into complicated divergences which lead finally to rosettes of leaves radiat- ing on all sides. This occurs, in Aloe ciliaris^ latifolia, brachyphylla, Lingua, nigricans, and Serra. 152 shows the transverse section of a shoot of the last-named species; first six leaves are arranged alternately in two rows with a constant diver- genc
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectbotany, bookyear1882
