REPRODUCTION AND DISPERSAL 823 



gametophyte; here gametophytes but not gametes appear to be necessary 

 for sporophyte development. In Coelebogyne (one of the seed plants) 

 certain cells of the sporophyte nucellus are able to develop into embryos, 

 the entire gametophytic generation thus being unnecessary for seed 

 production. Apogamy is now recognized in a number of seed plants, 

 as in Euphorbia, Allium, Elatostema, and Balanophora; in Coelebogyne, 

 in Balanophora, and in Euphorbia dulcis there is no necessity for 

 sexual fusion; in Elatostema there is no egg, and in Balanophora 

 globosa even the staminate flow^ers are wanting. Apospory is illustrated 

 in certain ferns, in which the gametophytic generation may develop 

 from sporangia (as in Asplenium) or even from vegetative parts of the 

 leaf (as in Polystichum) . There are some varieties of ferns and of seed 

 plants which exhibit both apogamy and apospory, their reproduction 

 being wholly vegetative. The chief advantages of apogamy and of 

 apospory would appear to be that they eliminate the disadvantages of 

 alternating generations. 



Parthenogenesis. — The development of a gamete into a plant without 

 fusing with another gamete is known as parthenogenesis. The egg is 

 much more likely to develop parthenogenetically than is the sperm, prob- 

 ably because of its greater size and more abundant food supply.' In 

 such forms as Ulothrix, where a gamete almost indifferently either may 

 fuse with another gamete or develop independently, or in Ulva, where 

 small gametes commonly fuse and certain of the larger gametes usually 

 develop without fusion, parthenogenesis probably represents the retention 

 of a primitive character; perhaps the same is true in Zygnema (fig. 112). 

 The significance is quite otherwise in Saprolegnia, where parthenogenesis 

 is accompanied by all stages in the abortion of the male organs, from 

 almost complete development to the entire absence of the antheridium. 

 Other plants in which parthenogenesis has been reported are Chara 

 crinita, Marsilea, Thalictrum, Alchemilla, Wikstroemia, Hieracium, 

 Antennaria, and Taraxacum, the last six being seed plants, and the 

 final three being Compositae. In these cases also parthenogenesis 

 undoubtedly involves the loss of a character formerly present; in Wik- 

 stroemia and Hieracium the pollen often is imperfectly formed or im- 

 potent. Parthenogenesis occurs in many animals, as in rotifers and 

 in a number of insects and crustaceans. Like apogamy, partheno- 



' However, male parthenogenesis has been reported in the brown alga, Ectocarpus 

 sihcuiosus, though here the sperms are relatively large and the plants into which they 

 develop relatively small. 



