DEGENERATE SEXUAL REPRODUCTION. 



175 



2 sp.) and a beetle {Gastrophysa) ', some coccus-insects and Aphides; 

 certain saw-flies [Tenihredinidcc) and gall-wasps (Cj/^z^/V/^?), are normally 

 parthenogenetic. In the butterflies just noticed, the males seem to dis- 

 appear for a stretch of years, and the species gets on without them. The 

 male of Psyche helix is very rare, and was for long unknown. When the 

 males are developed in Solenohia ti-inqnetrella, it is interesting to notice 

 that they may predominate in numbers over 

 the females. A whole brood may be male ; they 

 are brought back with a rush. About a score 

 of moths, including the silkmoth {Bomhyx 

 mori) and death's-head {Sphinx atropos) have 

 been known to exhibit casual parthenogenesis ; 

 but the beetle above noticed stands alone. 

 Bassett, Adler, and others, have demonstrated 

 an interesting alternation of parthenogenesis 

 and ordinary sexual reproduction in numerous 

 gall-wasps. Forms which had been regarded 

 as quite distinct, and had received different 

 generic titles, have been shown in about a score 

 of cases to be merely the parthenogenetic and 

 normal forms of the same insects. From a 

 winter gall the parthenogenetic form emerges 

 which produces a summer gall. In this a sexual 

 form is produced, which eventually gives rise 

 to the winter gall. 



§ 4. Parthenogenesis in Plants. — The pas- 

 sive bias is so strong in plants, that it is easy 

 to understand the rarity of parthenogenesis. 

 The egg-cell which develops of itself must re- 

 tain the stimulus which the male element in 

 other cases supplies. It is natural, then, that 

 what predominates in the active rotifers should 

 be uncommon in the sleeping plants. In some 

 of the flowering plants, what looked like par- 

 thenogenesis has repeatedly been described, 

 especially in regard to a native of New PIol- 

 land, known as Cixlebogym. When cultivated 

 in Europe, the male flowers degenerate, and 

 according to Braun and Hanstein disappear. 

 Yet fertile seeds are produced. Karsten found, 

 however, that stamens often persisted ; while 

 Strasburger has shown that what developed 

 were not true egg-cells, but adventitious growths 

 from cells outside the embryo-sac. The same 

 is true of some other cases. Dr A. Ernst has 

 recently described what he calls true partheno- 

 genesis in a Menisperm found by him in Caracas, 

 and named Disciphania Ernstii. "Female plants, which bore no male 

 flowers, and which were grown perfectly isolated where there was no pos- 

 sibility of the access of pollen from another plant, produced in three succes- 

 sive years an increasing number of fertile fruits." 



In the lower plants, however, there is no doubt on the subject. 



Owen's figure of the Genera- 

 tions of Aphides. At the 

 base an individual arises 

 from a fertilised egg-cell ; 

 this gives origin partheno- 

 genetically to a brood, and 

 so on through a succession 

 of generations. At the top 

 the male and female forms 

 reappear, and sexual re- 

 production returns. At the 

 side an earlier appear- 

 ance of sexual forms is 

 suggested. 



