478 BOTANY OF THE LIVING PLANT 



as " somatic parthenogenesis," which impUes that the embryo springs from the 

 ovum, but tire ovum was itself diploid. A like condition has been seen in 

 JMarsilia Drammondi, and in Atliynum filix-foemina, var. clarissima. In 

 such cases again no fertilisation is necessary to arrive at the diploid state. 

 A more rare condition is that where an egg that is really haploid develops 

 as though it had been fertilised. This is not known in Seed-Plants, but it 

 has been found to occur in Cliara crinita. It is described as "generative 

 parthenogenesis." 



Contrasted with these cases of apomixis, where no sexual fusion occurs, 

 are A-arious conditions which ma}' be ranked as substituted sexuality. Here a 

 nuclear fusion is seen, with consequences like those following on normal 

 syngamy. But the nuclei involved are not produced in the normal way. 

 Examples have already been described for Nephrodiutn pseudo-mas, var. poly- 

 dactyhini (p. 350, Fig. 294) ; and in that unusual type of nuclear association 

 which occurs in the formation of the fruit in the Uredineae (p. 448, Fig. 384). 

 In the latter the association is ultimately followed by nuclear fusion, 

 which thus, though deferred, takes its place in the cycle. The fact that 

 similar nuclear fusions precede the formation of ascospores (p. 430) and of 

 basidiosporcs (p. 441) suggests that such methods are probably wide-spread 

 among Fungi. The nuclei involved in such cases spi-ing not from any widely 

 distinct sources, but from cells closely related in position and in origin. It is 

 remarkable that these and other irregularities in the sexual cycle are found 

 commonly among Plants represented by very numerous closelj' related forms. 

 If their numerous species and varieties have resulted from mutation, then it 

 would appear that excessive mutation may have had some influence in pro- 

 ducing those irregularities. In organisms which show mutation freely, the 

 Slendelian sifting out, and preservation of each heritable mutation, would be 

 a less vital matter than it is in more stable forms. This may in some degree 

 account for such deviations from the normal sexual reproduction as have just 

 been described. 



