380 INTRODUCTION TO EMBRYOLOGY OF ANOIOSPERMS 



Yet another method of overcoming the difficulty caused by an 

 extremely slow growth of the pollen tube would be a direct introduc- 

 tion of the pollen grains into the ovary. Somewhat reminiscent 

 of the artificial insemination practiced in animals, this technique 

 has not so far been perfected for plants. That it is entirely possible 

 to do so, however, is indicated by some experiments of Dahlgren 

 (1926), who succeeded in bringing about a fertilization of the ovules 

 of Codonopsis ovata by this method. More recently, Bosio (1940) 

 tried some intraovarial pollinations in Helleborus and Paeonia. He 

 emasculated the flowers and either removed the stigmas or painted 

 them with celloidin. Then an incision was made in the ovary and 

 the pollen grains introduced into it artificially. In Helleborus the 

 germination was inadequate to cause any fertilization, but in Paeo- 

 nia several ovules were fertilized yielding viable seeds. In explana- 

 tion of this difference in the behavior of the two genera, the author 

 says that the pollen of the Ranunculaceae requires for its germina- 

 tion a sugary medium, with a pH close to neutral. Within the 

 ovary of Helleborus there is no free liquid, and the pH of the cells 

 lining it is about 4; in Paeonia, on the other hand, the cells at the 

 base of the ovarian cavity secrete an abundant liquid, which has a 

 suitable concentration of sugar and a pH of about 6. Germination 

 of the pollen, therefore, fails in the former but is quite successful 

 in the latter. In nature it occurs in both cases, since the stigma 

 fulfils the required conditions. 



In addition to these mechanical devices for bringing the pollen 

 grains or pollen tubes in close proximity to the ovules, it seems 

 possible that the same result may sometimes be achieved by the 

 application of suitable chemical substances to either pollen grains 

 or stigmas. From experiments in vitro, P. F. Smith (1942) has 

 shown that 3-indoleacetic acid and 3-indolebutyric acid, in concen- 

 trations of one in a million, appreciably stimulate the germination 

 of the pollen as well as the rate of elongation of the pollen tubes. 

 More recently, Addicott (1943) has reported that several substances 

 including vitamins, plant hormones, pyridines, and purines are able 

 to bring about similar effects. He also states that germination of 

 pollen and the subsequent growth of pollen tubes are not necessarily 

 related phenomena and that one can be stimulated independently 

 of the other. In his experiments, inositol increased the germina- 

 tion of Milla pollen up to 90 per cent over that of the controls 



