422 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



the ovuliferous scale. 4 In Gnetum germination frequently takes 

 place in the inicropylar canal at some distance from the apex of 

 the nucellus. 



The angiosperms differ from all known gymnosperms in having a 

 closed carpel whose upper portion becomes differentiated into a 

 style and stigma. Pollen grains never land directly on the nucel- 

 lus but a considerable distance away from it on the tissues of the 

 stigma, and the pollen tubes have to grow all the way down through 

 the style before they can reach the ovules. 



This difference, which is very significant, seems to be bridged to a 

 certain extent by Johri's (1936) discovery of pollen grains in the 

 stylar canal and ovary of Butomopsis. Here the style is a hollow 

 structure which remains open at its upper end, so that the ovary 

 is in direct communication with the exterior. As a rule the pollen 

 grains germinate on the stigma as in other angiosperms and the 

 pollen tubes travel down the walls of the hollow stylar canal to 

 the ovary, but in one carpel a row of six pollen grains was found 

 within the stylar canal, five of them having been seen in a single 

 section (Fig. 215A,B). In another carpel eight pollen grains were 

 seen (Fig. 215C) and in a third there was a pollen grain at the junc- 

 tion of the stigma and style (Fig. 215D). In a fourth there were 

 two pollen grains in the stylar canal, the upper of which had ger- 

 minated in situ (Fig. 215E). Finally one case was seen in which a 

 pollen grain had germinated on the surface of an ovule (Fig. 

 215F,G). 



Intracarpellary pollen grains have since been found in some other 

 angiosperms, notably Trillium, Ottelia, Fritillaria, Amianthium, and 

 Erythronium, 5 although it is not known if any of them germinate 

 to form pollen tubes which take part in fertilization. In any case 

 this is a remarkable phenomenon, which is comparable only to the 

 condition in the Caytoniales, in which the carpel is closed at ma- 

 turity but pollen grains are nevertheless found in the micropyles 

 of many ovules (Harris, 1933, 1940). Presumably the carpel was 

 open at its upper end at the time of pollination and the micropyles 

 of the ovules were connected to the stigma by means of narrow 

 canals through which the pollen grains were drawn in by some 

 suction mechanism. In conclusion Harris says: "There is virtu- 



4 For further information, see Doyle (1945). 



6 Unpublished observations made bv the author's pupils. 



