238 FUNDAMENTALS OF CYTOLOGY 



appears to be little if any significant correlation of chromosome number 

 and form on the one hand with structural complexity and the major 

 divisions of the plant and animal kingdoms on the other. It is true that 

 certain great groups show definite tendencies: most fungi have very low 

 numbers; true ferns have high numbers; plants of the hly family and 

 amphibians have large chromosomes. It seems evident, however, that 

 the major evolutionary fines are distinguished by kinds and assortments 

 of genes rather than by the number, size, and form of chromosomes. 



Chromosome Morphology. — Species may differ in the number of their 

 chromosomes, or in their visible morphology, or in both. In plants or 

 animals with very small or very numerous chromosomes it is often 

 impossible to use chromosome form as a character in the study of relation- 

 ships. The value of the character increases as the chromosomes become 

 larger and fewer. The genus Crepis has long been one of the most 

 valuable to the cytologist, for although the chromosomes are of only 

 medium size they are well differentiated morphologically and occur in 

 genomes composed of very few members. In a majority of the species 

 the gametic number is 4, while in most of the rest it is only 5 or 6. The 

 characteristic morphology of these chromosomes in several of the species 

 with the lower numbers is illustrated in Fig. 71. It is plainly evident 

 here that each species is characterized by a chromosome complement 

 that is not only Hke those of its relatives in general features but also 

 unlike them in certain respects. The particular kind of chromosome 

 complement characteristic of any individual or group of related organisms 

 is called a karyotype. The diagrammatic representation of a karyotype 

 is an idiogram. 



Another example of distinctive karyotypes in a genus of plants is 

 afforded by Plantago (Fig. 170). The chromosome complements in this 

 genus are composed of members differing in size as well as in morphology 

 and number. It is their morphology that makes it possible to decide 

 that the complement of 18 somatic chromosomes in one of the species 

 mentioned on page 235 consists of two genomes of 9 members each rather 

 than three genomes of 6, and that each genome of 9 may in turn represent 

 a combination of two basic genomes with 4 and 5 members. 



In related genera the morphology of the chromosomes may, like their 

 number, be strikingly similar or unlike in various degrees. In maize and 

 its relatives the densely staining knobs characteristic of the chromo- 

 somes of these plants aid in cytological comparisons of the various genera. 

 The genomes of maize (Zea mays), annual teosinte (Euchlaena mexicana) 

 from southern Guatemala, and gamagrass (Tripsacum fioridanum) are 

 represented as idiograms in Fig. 171. Inspection of these idiograms 

 shows that maize and gamagrass differ in chromosome number, knob 

 position, and arm ratio. Maize and Mexican annual teosinte are very 



