POI.YEMBRYONY 



467 



from the embryos of Dicotyledons in the relative positions of the 

 cotyledon and j^lumule. Although the cotyledon ai^parently 

 arises laterally, it soon becomes terminal and the plumule appears 

 to develop on the side of the em- 

 bryo {Fig. 413). 



Parthenogenesis. — Partheno- 

 genesis, which is the develop- 

 ment of an embryo from a sup- 

 posedly unfertilized egg, occurs 

 in a number of Angiosperms. 

 In the Dandelion {Taraxacum), 

 Meadow Rue {Thalidrum) , Ever- 

 lasting {Antennaria) , Apples, 

 Pears, Quinces, and a few other 

 plants parthenogenesis is known 

 to occur. In cases which have 

 been investigated cytologically, it 

 has been found that the mother 

 cell in the ovule omits the reduc- 

 tion division, and, therefore, the 

 cell which occupies the position of 

 an egg has the sporophytic num- 

 ber of chromosomes and fertiliza- 

 tion is not necessary. Since par- 

 thenogenetic plants show no re- 

 sults of crossing in the offspring 

 when cross-pollinated, partheno- 

 genesis may be a source of disap- 

 pointment to the plant breeder. 



Parthenocarpy. — Parthenocarpy is the development of fruit 

 without fertilization and is quite common among Angiosperms. 

 Bananas, seedless Oranges, and seedless Currants are familiar 

 examples of parthenocarpic plants. Sometimes Apples develop 

 without seeds, and some varieties of Cucumbers develop fruits 

 without pollination. 



Polyembryony. — In a few Angiosperms, of which one of the 

 Onions {AUium) is a notable example, a number of embryos ma}^ 

 be developed in the same embryo sac or around it. The syner- 

 gids and antipodals have been known to develop embryos, and 

 sometimes some of the cells of the nucellus around the embryo 



Fig. 413. — A inonocotyledon- 

 ous embryo as typified by that of 

 Corn. The cotyledon (c) appears 

 terminal and the plumule (p) as 

 arising from the side of the em- 

 bryo. 



