REPRODUCTION AND DISPERSAL 883 



may be that the various stimulating influences have as their chief r6le the establish- 

 ment of increased permeability. In most cases artificially parthenogenetic animals 

 die in an early stage of development, but in at least one instance mature sea urchins 

 have been secured by this means. 



Few similar experiments have been performed with plant eggs, though 

 parthenogenesis has been induced in Spirogyra and in Chlamydomonas 

 by growing ptants in concentrated (6 per cent) solutions of cane sugar; 

 in these experiments plasmolysis occurred, indicating the extraction of 

 water, as in Arbacia. Artificial parthenogenesis has been reported for 

 other plants, such as Protosiphon and Marsilea, high temperature seem- 

 ing to be the stimulating factor. The experiments on artificial partheno- 

 genesis seem to suggest that the role of the sperm is less that of a carrier 

 of necessary hereditary substance than that of a growth excitant, which 

 by physical or chemical means makes the egg permeable to the sub- 

 stances which bring about development. 



Sexuality in the fungi. The sexual relations of the fungi are very 

 suggestive of modifications resulting from saprophytic or parasitic modes 

 of life, although confirmatory experimental evidence is largely lacking. 

 In Saprolegnia and Achlya (figs. 155-157) there are all gradations be- 

 tween completely developed male sexual organs and the absence of such 

 organs. Some forms have apparently complete sexual organs but the 

 eggs develop parthenogenetically; other forms have antheridial tubes 

 which reach the egg but remain closed or merely pierce the oogonium 

 wall without reaching the egg; still other forms have no antheridial tube, 

 and some forms have no antheridium. There may be considerable 

 variation also within a given species; for example, antheridia are rarely 

 present in Saprolegnia Thureti, as often absent as present in 5. mixta, 

 and usually present in S. hypogyna; they are always present in S. 

 monoica. In no case are the female organs absent, so that Saprolegnia 

 forms a striking instance of parthenogenesis by reduction. 1 In the 

 zygomycetes there are gradations between heterogamy and isogamy ; 

 suggesting the evolution of the latter from the former by reduction, and 

 in the ascomycetes there appear to exist many stages in the reduction of 

 sexuality. In comparatively few fungi does there appear to be a fusion 

 of ordinary gametes, though a number of apparently modified forms of 



1 Cases of reduction are known also in animals; for example, some rotifers have small 

 and functionless males or none at all, and in some crustaceans (as Limnadia Hermanni) 

 and ostracods (as Cypris reptans) only females are known, though they still retain the 

 sperm sac. 



