PARTHENOGENESIS : SEX DISTRIBUTION 329 



wounds — callus formation, cork formation, and the produc- 

 tion of roots and other new organs — to the action of special 

 substances produced in the wounded cells, which he calls 

 " wound hormones." He supposes that the stimulus to 

 parthenogenctic development lies in wound hormones 

 produced as the result of the abnormal divisions and dis- 

 integrations to which we have referred. He has in fact 

 been able to initiate the development of the egg cell, and 

 to induce the formation of nucellar embryos by mechanical 

 injury, squeezing and pricking, of the ovules of (Enothera. 

 He extends his theory to include the stimulus to develop- 

 ment by normal fertilisation, the wounding being here the 

 result of the entrance of the sperm. Ernst's theory, then, 

 which has evidently much to recommend it, supposes that 

 those abnormal modes of reproduction in which the sexual 

 fusion is omitted occur not in normal species, but in inter- 

 specific hybrids. There are three possibilities in regard 

 to the fate of these hybrids. The abnormality may become 

 permanent, a new mode of reproduction. It may result 

 in the ultimate disappearance of the hybrid. It may 

 disappear, being replaced by normal reproduction, and a new 

 and normal species may thus arise. 



§ 5. Sex Distribution in Flowering Plants 



The " essential " organs of the flower are the stamens 

 (collectively the andrcecium) and the carpels (collectively 

 the pistil or gynoecium) ; both are, of course, sporophytic. 

 Each stamen has four embedded sporangia which give rise 

 to numerous microspores, the pollen grains. The carpel 

 contains one or more stalked and integumented sporangia, 

 the ovules, in each of which a single functional megaspore, 

 the embryo sac, is produced. It is customary to refer to the 

 stamens as male, and to the carpels as female organs. This 

 is done as a matter of convenience and tradition. It is not 

 really legitimate, for these are not sexual reproductive 

 organs. They are organs of the sporophyte, and produce 

 spores within which the sexual gametophytes are formed. 



