[Chap. XXXVII HEREDITY IN PLANTS 451 



sperm, it has the 2n number of chromosomes. Starting then with the 

 f ertihzed egg, vegetative cell division ( mitosis ) results in an exact dupli- 

 cation of tlie 2n number of chromosomes in every cell of the embryo, 

 root, stem, leaf, sepal, petal, stamen, anther (including microsporo- 

 cytes ) , pistil, and ovule ( including the megasporocyte ) . Thus the cycle 

 is completed. Reduction division in the microsporocyte and megasporo- 

 cyte initiates another cycle. 



Unless some irregularity occurs, this chromosome cycle is repeated 

 annually in all plants that reproduce once each season. The 2n, or dip- 

 loid, number of chromosomes occurs in all cells of the plant with two 

 exceptions: (1) the microspores, pollen grains, pollen tubes, mega- 

 spores, and embryo sacs ( previous to nuclear fusions ) in all of which the 

 nuclei have the n, or monoploid,^ number of chromosomes; and (2) the 

 cells of the endosperms of seeds which have the Sn, or triploid, number 

 of chromosomes.^ Irregularities in the occurrence of the n and 2n num- 

 ber of chromosomes are described in Chapter XXXIX. 



The pairing and segregation of chromosomes during reduction division. 

 The pairing and subsequent distribution (separation and segregation) 

 of chromosomes during reduction division are so important in the trans- 

 mission of hereditary factors in sexual reproduction that they are singled 

 out for further emphasis. The two chromosomes of each pair on the 

 spindle of the cell are often referred to as homologs and as synaptic 

 mates, and the pairing is called synapsis. One of these two chromosomes 

 came from the sperm of the pollen parent, the other from the egg of the 

 ovule parent. They are usually similar in appearance and contain many 

 genes that influence the development of the same kinds of traits such as 

 flower color and height. 



These homologous chromosomes are all present in the fertilized egg, 

 in each cell of the vegetative tissues of the plant, and in every micro- 

 sporocyte and megasporocyte. They become segregated in separate cells 

 following their migration to the opposite poles of the cell spindle during 

 reduction division, and do not get together in the same cell again until 

 fertilization occurs. Hence each microspore and sperm, and each mega- 

 spore and egg contains only one of the homologs of each pair. 



The pairing of chromosomes is remarkably constant. That is, the 

 same homologs pair during every reduction division in a plant, and in 



2 Also called haploid by many authors. 



^ In those plants in which the fusion nucleus is formed by the union of more than two 

 nuclei, the number of chromosomes is correspondingly increased in it and in the subsequent 

 triple-fusion nucleus and endosperm cells. 



