328 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 4 



with &, and so on through the ^-pair. As division continues, the mem- 

 bers of each pair of chromosomes separate, moving to opposite poles, 

 with a wall eventually separating the two groups. The two new cells 

 have half the number of chromosomes found in the mother cell. If 

 this occurs in the anthers, the end result will be male gametes; if in 

 the ovule, the egg cells will develop. The diploid phase starts again 

 upon fusion of a sperm and egg. 



However, if one of the sets of 11 chromosomes forming the egg 

 were sufficiently different that we would have to label them p, q, r, s, t, 

 u, V, w, X, y, s, pairing of the chromosomes previous to the reduction 

 divisions usually would not take place, and the cells would abort. Or- 

 dinarily no fruit or seeds can develop. This is what apparently hap- 

 pens in the common day lily {TI emerocallis fulva^ clone Europea) (pi. 

 6, fig. 25). This day lily was introduced from Europe perhaps over a 

 century ago. Although no fruits or seeds ever develop, it propagates 

 readily from the rhizomes. It is found thoroughly established along 

 roadsides and railroads in many parts of the country — dumped there 

 as waste from someone's garden. 



The seedless fruit varieties of orange, grapefruit, pineapple, banana, 

 grapes, and persimmons are further examples of sterility because of 

 irregularities in gamete formation, but differ from the day lily in that 

 the pistil of the flower, although sterile, does not abscise, but continues 

 to grow, and matures as a seedless fruit. Such plants cannot survive 

 in the wild without the aid of man. 



As stated previously, all cells of the vegetative parts of a seed 

 plant ordinarily are diploid, established by the fusion of the two 

 monoploid gametes. The diploid nmnber is maintained by the mitotic 

 cell divisions. In this kind of division, the chromosomes set is dupli- 

 cated in every division. Occasionally an accident happens which pre- 

 vents the formation of a new cell plate and the duplicated chromo- 

 somes remain together in one cell. The diploid number is doubled by 

 this method and the cell is a tetraploid. If this accident should occur 

 as the zygote divides, every cell of the embryo, seedling, and mature 

 vegetative individual will be tetraploid. If it occurs in one cell of 

 a two-celled embryo, the shoot may be tetraploid and the root diploid, 

 or vice versa. If it occurs in a somewhat older embryo, only that por- 

 tion of the shoot derived from the original tetraploid cell will have 

 the 4n number of chromosomes with other parts remaining diploid. 

 This tetraploid state may occur spontaneously or it may be induced 

 by treatment with colchicine and a few other reagents. Tetraploid 

 plants, in general, may have thicker leaves, and larger stems, flowers, 

 fruits, and seeds. Such plants are usually slower in growth, and con- 

 sequently are later in flowering and fruiting. 



Somewhere in western Mexico, the tetraploid condition developed 

 in the annual teosinte mentioned previously. The tetraploid plant has 



