302 



A TEXTBOOK OF BOTANY 



(Ch. VI, 7 



A very special and rare form of asexual reproduction is 

 found in parthenogenesis, i.e. the development of an egg 

 cell to a plant without fertilization. This is known in 

 relatively few plants, which include especially the Composite 

 family, though it is freciuent in the reproduction of Insects. 

 Closely related thereto are cases of polyembryony, in 

 which cells of the embryo sac other than the egg cell de- 

 velop into apparent embryos, 

 which, however, are really 

 vegetative structures of the 

 nature of branches. Both 

 parthenogenesis and poly- 

 embryony are too rare to 

 play any appreciable part in 

 plant reproduction as a whole, 

 and have their chief interest 

 as showing the plasticit}' of 

 natural processes. 



A question must arise at 

 this point as to whether any 

 differences exist between the 

 sexually and ascxually pro- 

 duced offspring of the same 

 plant. It seems clear that 

 in general the asexually pro- 

 duced individuals arc less 

 variable in their characters 

 than the sexuallj' produced, 

 although there are notalile exceptions. At all events it is cus- 

 tomary among gardeners to employ asexual propagation when 

 they wish to retain a variety true to some valued character, but 

 to use sexual or seed reproduction when trying for novelties. 



Fig. 214. — The .spore cases and 

 spores of a common Mold : X 3S. 

 (From Strasburger.) 



7. The (_)ri(:;in and Si(ixiFic-\.NCE of Sex 



In the preceding sections, the two fusing sexual nuclei, 

 the male and the female, have lieeii treated as functionallv 



