. A textbook of botany for colleges and universities ... Botany. REPRODUCTION AND DISPERSAL 833 related species) ^; in some species pollen is essentially impotent on the stigma of the flower in which it was produced (p. 854). Pollen grains usually retain their vitality for a number of days, but those of Hibiscus Trionum live scarcely more than three days, while those Of some species (as the date palm) may live for several months, especially if kept dry. Usually pollen grains that have been moistened and subsequently dried die quickly, but some pollen is so resistant that submergence for a num-
. A textbook of botany for colleges and universities ... Botany. REPRODUCTION AND DISPERSAL 833 related species) ^; in some species pollen is essentially impotent on the stigma of the flower in which it was produced (p. 854). Pollen grains usually retain their vitality for a number of days, but those of Hibiscus Trionum live scarcely more than three days, while those Of some species (as the date palm) may live for several months, especially if kept dry. Usually pollen grains that have been moistened and subsequently dried die quickly, but some pollen is so resistant that submergence for a num- ber of hours does not impair its vitality. The pollen of vernal flowers is especially resistant, not only to moisture, but also to low Figs, 1159, 1160. — The dioecious "wind-pollinated flowers of the box elder {Acer Negundo): 1159, tascides of drooping staminate flowers borne on long stalks or pedicels (p); note the prominent anthers (a); 1160, ascending racemes of pistillate flowers from another tree; note the perianth, consisting only of a calyx (c), and also the two prominent stigmas (i); note also the transition between the bud scales (&) and the ordinary foliage leaves (/). the intermediate leaves having a prominent fiattish petiole (0) and a small trifoliate blade (6'). In general, the stigmas are more sensitive to harmful factors than are the pollen grains. Pollen tubes usually take a more or less direct course toward the ovary. Commonly the central region of the style is composed of delicate elongated cells, or sometimes, even, it is hollow, so that the direct course is the easiest; in the grasses, however, the region traversed by the pollen tube seems no more easily penetrable than do the adjoining tissues. After leaving the style and entering the ovary, the pollen tube commonly ^ Often germination, but not the later stages of development, may take place on the stigmas of unrelated plants; the pollen tubes of Ranunculus, a dicotyl, have been
Size: 2181px × 1146px
Photo credit: © Central Historic Books / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
