202 INTRODUCTION TO EMBRYOLOGY OF ANOIOSPERMS 



plasmic sheath around the male nuclei of Orobanche, although they 

 could not follow it up to the time of fertilization. The inability of 

 Breslavetz (1930) to find mitochondria and plastids around the 

 male nuclei also cannot be considered as an absolute proof of the 

 absence of the plasma layer; in fact her reference to certain lighter 

 areas around the male nuclei is a fairly good indication of the pres- 

 ence of the male cytoplasm. Gershoy's observations, of which 

 only a preliminary account has so far appeared, seem to be contra- 

 dicted by those of West (1930). 



In conclusion, it may be well to emphasize that until a few years 

 ago the male gametes of angiosperms were usually considered to be 

 naked nuclei. Recent studies on the subject leave no doubt, how- 

 ever, that the cytoplasmic sheath remains intact at least for the 

 period during which the male gametes are in the pollen tube. 4 It 

 would not be surprising, therefore, if with some further improvement 

 in technique it may be possible in future to trace the fate of the 

 male cytoplasm in a more precise manner than has been done up 

 to this time. Finn (1935, 1940, 1941) has suggested that in order 

 to decide the point with certainty the whole series of events should 

 be studied in living material, but this seems to be impracticable with 

 most plants, as the embryo sac is enclosed in several opaque layers 

 which interfere with a direct and detailed observation of its con- 

 tents. 6 The only alternative is to look for some suitable material 

 in which (1) fertilization stages may be found abundantly, (2) the 

 process does not take place too rapidly, and (3) the gametic cells 

 not only are fairly large but also respond more favorably to our 

 staining methods. In addition every effort must of course be made 

 to develop new methods of fixing and staining which would be more 

 suitable for a study of the contents of the embryo sac at the time of 

 fertilization. 



Multiple Fusions and Polyspermy. As is well known, usually 

 only one pollen tube enters an ovule. Compton (1912) saw an ovule 

 of Lychnis with two embryo sacs, each of which had been penetrated 



4 See also p. 168. 



5 In a few genera like Torenia and Utricularia, where the embryo sac protrudes 

 out of the micropyle, it may be possible to make direct observations on living 

 material. In certain others, like Monotropa and some members of the Orchidaceae, 

 the seed coat is thin and transparent. No recent studies have, however, been made 

 on the phenomenon of fertilization in any of these plants. 



