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 temperatures. 



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 seen 

 penetrating to the micropyle of SciHa, a monocotyl. It may be noted in this connection 

 that the sperms of ferns swim mto the archegonia of many species indifferently, but that 

 fusion with the egg takes place only in the same or in a closely related species. 



