478 TEXTBOOK OF BOTANY 



number of chromosomes when compared with gametes of the ancestor 

 of the tetraploid plant. If they unite they form a zygote with the 4n 

 number of chromosomes and thus the tetraploid condition is perpetuated. 

 But if a diploid (2n) gamete unites with a monoploid (n) gamete, a 

 triploid (3n) zygote is obtained. Certain irregularities, however, may 

 occur during reduction division, and some of the progeny may not have 

 exactly 4n or 3n number of chromosomes. 



Tetraploid plants that have been obtained by experimental methods 

 usually have larger cells, stems, leaves, flowers, and seeds than their 

 diploid ancestors, though this increase in size does not always occur. A 

 slower growth and a longer vegetative period are more generally char- 

 acteristic of them than increase in size. Many irregularities and various 

 degrees of sterility occur. 



Monoploid tomato plants and Jimson weeds that develop from mono- 

 ploid unfertilized eggs (parthenogenesis) are smaller and less vigorous 

 than the parent diploid plant. Those that have been studied are sterile 

 unless they produce gametes without reduction division. All such 

 gametes have the same chromosome complement and it is identical with 

 that of the vegetative cells of the plant. If self-fertilization were to occur, 

 absolutely homozygous individuals would be obtained and perpetuated 

 until some change occurred in a gene of one or more of the chromosomes. 

 In later chapters we shall see that the formation of absolutely homo- 

 zygous individuals is a common occurrence in some species of mosses 

 and ferns. 



Other aberrations of chromosomes noted above, such as a gain or a 

 loss of a chromosome or of a fragment of a chromosome, are, of course, 

 accompanied by changes in development of the plant, depending upon 

 whether the cells gained or lost certain genes and their potentialities. 

 These changes may be more striking than those that are merely the result 

 of the doubling of the number of similar chromosomes. 



We shall see presently that tetraploid (4n) plants, octoploid (8n) 

 plants, and 16-ploid plants may originate in another way; but certain 

 other points may be considered first. It is now known that species of 

 many genera of plants differ from each other by multiples of some basic 

 number of chromosomes. For instance, the different species of wheat 

 have either 7, 14, or 21 pairs of chromosomes. Similar series of multiples 

 of seven also occur in the different species of oats, cinquefoil, tall 

 meadow rue, and certain other genera of plants. These facts are suffi- 

 cient evidence that the number of chromosomes alone is not enough to 



