678 General and Applied Biology 



known; also chrysanthemum species with 18, 36, 54, 72, and 90 chromo- 

 somes. The regularity of these series of numbers (multiples of a lower 

 number) suggests the origin of new species by doubling the chromosomes 

 or by adding a single set such as found in a sperm or egg. Individuals 

 with three or more single sets of chromosomes in their body cells are 

 known as polyploids, and the condition is known as polyploidy. 



In animals such series of chromosome numbers are encountered less 

 frequently than in plants, and polyploidy has rarely been induced in ani- 

 mals by artificial methods. One explanation is that higher animals have 

 their sexes separate, while most plants are monecious (hermaphroditic). 

 If the sexes are separate, the doubling of chromosome numbers can 

 occur only in case it takes place in males and females which subsequently 

 cross. In addition, the doubling of chromosomes may lead to irregularity 

 in the distribution of the sex chromosomes (X and Y chromosomes) 

 which may cause sterility. In animals, new species probably originate by 

 the slower method of gradual divergence by the accumulation of gene 

 differences rather than by chromosome doubling. 



Plants which have doubled numbers of chromosomes usually are larger, 

 have thicker stems, thicker, broader leaves, darker green color, larger 

 flowers and seeds; they develop more slowly, take longer to mature, and 

 are often more hardy than the original plants. Many new plant types 

 have been produced which are superior economically because of their 

 doubled chromosomes. Tomatoes with doubled chromosomes have been 

 produced; they contain about twice the usual quantity of vitamin C. 



Chromosomes (Figs. 332-335) and their genes play important roles 

 during the process of mitosis whereby the determiners for future traits 

 are accurately duplicated during the division of cells. If genes were not 

 accurately duplicated during mitosis, the resulting daughter cells might 

 not possess their necessary hereditary materials from which future traits 

 could be developed. Much valuable information regarding the internal 

 structure of chromosomes has been secured by a study of the stained 

 cells during mitosis. 



During the so-called resting stage of a cell the chromatin is in the 

 form of long thin, granular threads (chromatin strands) which even- 

 tually will shorten and thicken to become chromosomes^ the number, 

 size, and shape of which are specific for each species of plant or animal. 

 During the prophase stage of mitosis each chromosome appears to pos- 

 sess a pair of thin, fiberlike (often coiled) chromonemata (kromo-ne'- 

 ma ta) (Gr. chroma, color; nema, thread) or gene strings. Each 

 chromonema contains a specialized region known as the centromere (sen'- 



