Pollen Tube Growth and Control of Gametophytic Selection in Cocklebur, a 25-Chromosome Datura

1. Pollen tube distributions were plotted from 12-, 16-, and 18-hour tests, using the pollen of the primary (2n + 1) Cocklebur germinated and grown on the pistils of 2n plants. These tests were compared with similar tests of pollen tube growth of normal plants in line 1a. 2. Further evidence is given from the results of ordinary male back-crosses of Wedge to four lines, indicating that the latter is a secondary (2n + 1) type related to Cocklebur. Pollen tube distributions obtained from the pollen of Wedge are also plotted. 3. The pollen tube distribution curves indicate that Cocklebur may be transmitted through the pollen under conditions in which the processes of gametophytic selection are controlled experimentally. Likewise, the distribution curves indicate that Wedge may not be transmitted through the pollen in line 1A. 4. The correctness of the interpretations gained from pollen tube distribution curves was tested genetically in the case of Cocklebur, by combining three other experimental methods: the effects of restricted pollinations; the effects of excision of styles; and the separation of the seeds in the lower half of seed capsules from those in the upper half. Results from all experimental methods confirmed the interpretations gained from a study of pollen tube distribution curves. 5. The various methods which have been used by investigators at different times in a study of the effects of gametophytic selection are briefly discussed and compared. 6. The pollen tube distribution curves obtained from suitable tests enable us to recognize processes of gametophytic selection, and have the further advantage of enabling us to distinguish between at least five different conditions, all of which may lead to the complete or partial elimination of some of the classes of male gametophytes, resulting in modified ratios in the progeny. 7. The processes of gametophytic selection involved in the pollen transmission of Cocklebur may be controlled through a wide range. A threefold increase may be obtained by restricted pollinations, and a further increase, up to 65 per cent Cocklebur plants, may be obtained in a progeny by selecting the seeds to be planted from the lower parts of seed capsules, while complete non-transmission of Cocklebur may be obtained by properly timed excisions of styles.