424 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



ally no evidence to show what was the pollination mechanism of 

 primitive angiosperms but it seems by no means unlikely that it 

 may have been similar to what is postulated for Caytonia. Possibly 

 the style was originally an open canal along which the pollen was 

 conveyed to the ovules; a later stage would be the germination of 

 the pollen grain before it reached the micropyle, at first no doubt 

 at the bottom of the stylar canal, then at its middle and then at its 

 top. The final change to occur — the closure of the stigma — makes 

 It impossible for the pollen to pass down the stylar canal." 50 



The condition in Butomopsis is exactly what Harris expects in 

 his primitive angiospermous types and it seems likely that more 

 comprehensive studies on other angiosperms with open styles may 

 give some clue to a solution of the problem. Of considerable in- 

 terest in this connection are also the carpels of the new Fijian genus 

 Degeneria in which Bailey and Smith (1942) report that the carpel 

 is a conduplicate structure whose margins are not coherent but 

 tend to flare apart externally (see also Swamy, 1949). In most 

 cases the cleft-like opening becomes more or less occluded, owing to 

 the presence of numerous loosely interlocking papillae, but it is 

 not impossible that sometimes the pollen may have direct access 

 to the ovules. 



Endosperm. There has been a great deal of discussion regarding 

 the morphological nature of the endosperm of angiosperms, which 

 is commonly neither haploid nor diploid but triploid. 



Hofmeister (1858, 1859, 1861), in whose days neither syngamy 

 nor triple fusion had yet been discovered, considered the endo- 

 sperm of angiosperms to be a gametophytic structure whose growth 

 and differentiation remained arrested until the entry of the pollen 

 tube into the embryo sac. 



Following Strasburger's (1884) discovery of syngamy in angio- 

 sperms, Le Monnier (1887) put forth the view that the fusion of 

 the polar nuclei is also an act of fertilization, comparable to the 

 fusion of the egg and the sperm nucleus. He therefore regarded 

 the endosperm as a second embryo, modified to serve as food tis- 

 sue for the zygotic embryo. 



With Nawaschin's (1898) announcement of double fertilization, 

 emphasis shifted from the fusion of the polar nuclei to the partici- 

 pation of the second male gamete in this event. Nawaschin re- 



** See Baum (1949). 



