Q. What do you mean by Chromosome Number?
We are generally aware that the number of chromosomes in each cell of all individuals of a single species is constant. ,It is also established that the more closely related species are likely to have similar chromosome numbers while the more distantly . related ones shall have different numbers. Due to this' relative conservativeness, chromosome number becomes an important and frequently used taxonomic character. In addition, there is a very wide range of chromosome numbers. in the angiosperms from as low as 2n = 4 (in Haplopappus gracilis) ('Asteraceae) to as high as 2n = 530 (in Poo litterosa) (Poaceae). A large number of angiosperms have been analysed for their chromosome numbers, providing useful taxonomic information.
Many interesting ideas have developed from knowledge of chromosome numbers. For example, in the genus Festuca, different species have different chromosome numbers forming a mathematical series. The chromosome numbers are 2n = 14,28, 42, 56, 70, etc. From this information, a generalisation can be made, that different species may have some common basis. If we assume that these chromosome numbers are based on a common denominator called x (and x = 7), then we can consider the different species to have multiples of this number. This denominator or base number (x = 7) can be considered as the basic set of genetic information carried by a plant, and due to the multiplication of this basic genetic set, the eiiolution of different species has occurred. Such a series is said to be polyploid in which the basic number (x) is equivalent to the haploid number of chromosomes in a diploid species (i.e. x = n = 7). The other species would then be tetraploid, hexaploid, octaploid, decaploid, etc. respectively.