1918] Goodspeed-Davidson: Controlled Pollination in Nicotiana 433 



only the fertilization of an extremely small percentage of the normally 

 matured ovules and possibly the completion of the earlier stages in 

 embryo and endosperm development are necessary to make abscission 

 impossible. The question of the inhibitory stimulus provided by the 

 passage of pollen tubes down the style is still an open one. The fact 

 that premature pollination in tobacco* has been shown to cause abscis- 

 sion lends support to the supposition that stylar penetration is im- 

 portant. In this connection the delicacy of the balance between 

 stylar penetration or fertilization and abscission is again emphasized 

 in that before the normal maturation of the female sexual elements 

 pollination causes abscission, whereas at their complete maturity stylar 

 penetration by a few pollen grains or fertilization of a few embryo- 

 sacs is sufficient to inhibit such abscission. 



In our experience there is in Nicotiana a certain stage of develop- 

 ment of the seed capsule beyond which automatic abscission does not 

 take place and spontaneous abscission cannot be induced. The 

 explanation of this situation is found in the fact that mechanical 

 tissue is rapidly developed in the pedicel of the flower somewhat after 

 anthesis. A flower has only to pass the dangerous period when after 

 cell separation (abscission) there is not sufficient mechanical tissue to 

 hold it in position, to be retained permanently upon the plant. It is 

 a question in such cases as experiments 14 and 17 above if abscission, 

 induced by lack of normal seed development, may not occur after its 

 normal time of occurrence. If it does so occur the unbroken cuticle 

 and the more or less unaffected vascular bundle tissue both supported 

 by the mechanical cylinder, may, on the one hand, prevent the drying 

 out of the living but separated cells of the separation layer and, on 

 the other, furnish the necessary supplies of water and metabolic 

 ingredients to the developing seed capsule. 



The results obtained in these experiments on controlled pollination 

 appear finally to prove that no normal pollen is produced by the F^ 

 Tahacum-sylvestris hybrids. As noted above the flowers of such 

 hybrids (with the exception of the parthenocarpic Tahacum "Cuba" 

 hybrids) always fall when allowed to self -pollinate under bag. Many 

 attempts extending over a number of years have been made to secure 

 successful self-pollination artificially. A considerable excess of pollen 

 has been applied in a single and in a number of successive pollinations, 

 the stigma has been irritated with foreign substances prior to self- 



4 Hartley, C. P., Injurious effects of premature pollination, U. S. Dept. Agr., 

 Bur. PI. Tnd., vol. 22, 1902. 



