146 PLANT GROWTH 



ditions. However, plants grown in a good environment pro- 

 duce larger seeds, which supply more stored food to the 

 seedling resulting in a more rapid growth of the seedling. 



Many plants are quite sensitive to a location change. 

 For instance, corn may fail to grow well if the "seed" is taken 

 a hundred miles or less from the place in which it has been 

 grown. Plants that are highly successful in one garden may 

 fail in the adjoining garden because of the difference in the 

 soil or some other change in the environment brought about 

 by the gardener. 



The other type of change, polyploidy, involves the addi- 

 tion of a whole new set of chromosomes. Polyploidy is some- 

 times called mutation also, but in a strict sense mutation 

 refers only to gene changes. A duplication of the chromo- 

 some set is usually accompanied by an increase in the size 

 of the cells. The usual number of chromosomes, as we have 

 seen, is 2n. In polyploids, the number may be 3n (triploid) , 

 4n (tetraploid), 5n (pentaploid), 6n (hexaploid), and so 

 forth. Plants with an odd number of sets are not able to 

 have normal meiosis and are highly sterile. Sometimes 

 changes involving a single chromosome occur. This results 

 in plants with 2n + 1 or 2n — 1 chromosomes. A 2n + 1 

 plant is called a trisomic. A 2n — 1 plant is known as a 

 monosomic. The phenomenon is called aneuploidy. Tri- 

 somies and monosomies in general are semi-sterile. 



High temperature during the development following the 

 fertilization of the egg has produced polyploidy in cereals. 

 Wheat or rye is pollinated and kept in a constant tempera- 

 ture of 25° C. for a twenty-hour period, which is necessary 

 for the growth of the pollen tube to the egg cell and for 

 fertilization. While the first divisions of the fertilized egg 

 are taking place, the temperature is increased to 43° C. for 

 twenty to thirty minutes, after which it is returned to a nor- 

 mal greenhouse. This high temperature is so near the maxi- 

 mum that the plant can withstand, that it is best to keep a 



