68 ZOOLOGY 



Reduction 5. In the higher plants there is a peculiar complica- 



and f ertili- -"11 



zationinthe tlon whereby certain cells come to have more chromo- 

 ffcfwerin sor nes than the number normal for the species. The 

 plants pollen tube, which has developed from the pollen grain, 



brings to the ovary two sperm cells, one of which unites 

 with an egg nucleus which has the reduced number of 

 chromosomes. The duplex number is thus made up, 

 and so far the process is essentially like that observed 

 among animals. In the maturation of the egg nuclei, 

 division takes place as in animals, and part of the 

 chromosomes are rejected. Two of the particles come 

 together and produce a nucleus in which, apparently, 

 the full number of chromosomes is restored. This is 

 then fertilized by the second sperm nucleus, and the 

 resulting zygote has one and a half times the duplex 

 number of chromosomes. Thus if 4 is the simplex num- 

 ber and 8 is the duplex, then it will have 12. The zygote 

 so formed does not produce an embryo, but instead pro- 

 duces a quantity of undifferentiated cells, constituting 

 the endosperm. This endosperm serves as nourishment 

 for the embryo proper, or the plant into which it de- 

 velops. That there is a real process of fertilization in 

 the formation of the endosperm is proved by the phe- 

 nomenon called xenia, whereby the seeds show the in- 

 fluence of the pollen parent. This is especially notice- 

 able in corn (maize), where red grains appear on white 

 ears, when they have been fertilized by pollen from 

 plants carrying the factor for red. 



The above account is based on recent observations on 

 particular plants ; it is probable that it is essentially 

 true for all the higher flowering plants (angiosperms), 

 but there are doubtless differences in detail. 



6. The chromosomes are not all alike. They may 

 differ visibly in size or shape, but there are many 



