468 PARTHENOGENESIS. 



seems contrary to the economy of plants that the egg-cell, produced at great expense 

 of energy, and in a sense the culmination of a plant's activity, should wither away 

 unproductive. Plants with hermaphrodite flowers can, if denied crossing, resort 

 to autogamy. But this is of course impossible with dioecious plants; instead of 

 autogamy the formation of embryos in unfertilized oogonia and ovules is a 

 possibility open to them, whereby their outlay of material and energy shall not 

 be wasted. Dioecious plants, which are likewise annuals, are especially liable 

 to the danger of extinction in the absence of pollen and consequent fruit-pro- 

 duction; for them the death of the individual may connote the disappearance of 

 the species. Against such possibilities many precautions exist amongst plants, 

 notably the formation of offshoots or brood-bodies; the leafy shoots arising from 

 these structures preserve the plant from such a contingency. In the same way 

 we may regard the formation of brood-bodies in the ovules of dioecious plants 

 as a means contrived to prevent the extinction of the species. The fact that 

 brood-bodies are formed in the ovules of not a few dioecious plants to which 

 pollen has not ready access, supports this view. There has been a specimen 

 of the Californian bush, Ohione halimifolia (an Atriplex, Chenopodiacese), for 

 many years in the Vienna Botanic Garden. This plant is dioecious; the Vienna 

 plant bears only female flowers, and pollen is not accessible within hundreds 

 of miles. Its stigmas remain unpoUinated, and its ovules unfertilized. But as 

 the autumn draws in, the ovaries of this plant begin to swell, and the perianth 

 which ensheaths the ovary expands, and what appear to be fruits are formed. 

 But these fruits are what we call " deaf " ; no signs of an embryo are to be found 

 within. Thus, in this plant, no brood-body has been produced; it is impossible 

 to say whether or no, at some former period, this plant ripened brood-bodies in 

 its unfertilized ovules. Why the male plants of Chara crinita are absent from 

 the Baltic, and those of Gnaphalium al/pinwm from the Arctic regions, are puzzles 

 as yet unanswered. In Chara crinita it is only on the coast-regions that male 

 plants are wanting; inland, male and female plants grow side by side. Possibly, 

 climatic conditions and the vicissitudes to which our existing Flora has formerly 

 been subject have brought this about, but we lack the data for continuing the 

 discussion further. 



If, in the plants enumerated, parthenogenesis be but a special case of oflfshoot 

 formation, it is a matter of indifference which cells within the ovule are the 

 starting-points for the brood-bodies. In Codebogyne, in addition to the egg-cell, 

 other cells belonging to the wall of the embryo-sac are concerned in the production 

 of brood-bodies. Cells quite outside the embryo-sac can also initiate these off'- 

 shoots; in which case they project as little papillae into the cavity of the embryo- 

 sac, where they continue their development. In this way several embryos may 

 arise side by side, a condition which has been termed Polyembryony. This phe- 

 nomenon occasionally takes place in the ovules of hermaphrodite flowers, in which 

 a normal pollination and passage of pollen-tubes to the micropyle occur. This 

 IS the case in certain Liliacese, polyembryony having been observed in species of 



