496 i Flora Unveiled
trees are also surrounded by a modified cone consisting of a cup- shaped fleshy leaf scale called an
aril. When mature, the aril turns a bright red color.
- For more detailed descriptions of gymnosperm and angiosperm morphology, see Willis,
Kathy, and Jennifer McElwain (2014), The Evolution of Plants, second Edition. Oxford
University Press. - Chromosomes are strands of DNA and protein that encode the complete set of genes, or
genome, of an organism. In a typical nondividing cell, the chromosomes are loosely dispersed
throughout the nucleus in the form of chromatin. Human cells contain forty- six chromosomes.
If the unraveled DNA strands of the forty- six individual chromosomes were laid out end to
end, the total length of the genomic DNA (all forty- six chromosomes) inside the nucleus would
be about 6.5 feet. When 6.5 feet of dispersed DNA is packed into a nucleus that is only about 6
micrometers in diameter, it becomes impossible to see full- length strands of chromatin. Prior
to nuclear division, however, the chromatin condenses into tightly packaged, highly organized
bodies (chromosomes) that can be strongly stained by dyes for viewing under the microscope.
The name “chromosomes” literally means “colored bodies.” The aniline dye basic fuchsin, which
stains chromosomes magenta red, is among the most effective for staining chromosomes. - The second sperm cell is not functioning as a true gamete because it doesn’t fuse with the
egg cell to form a zygote; it fuses with the central cell to form an endosperm cell. Therefore the
term “double fertilization” is somewhat misleading. It is not yet known whether the two sperm
cells are identical and interchangeable or whether one is specialized to serve as the gamete. - Darwin, C. R. (1876), The Effects of Cross- and Self- Fertilisation in the Vegetable Kingdom.
John Murray. - Darwin, C. R. (1859), On the Origin of Species by Means of Natural Selection. A Mentor
Book, New American Library, p. 185. - Darwin, C. R. (1859), On the Origin of Species by Means of Natural Selection, third edition.
John Murray, p. 296. - Henig, R. M. (2000), The Monk in the Garden. A Mariner Book, Houghton Miflin.
- Quote from Ibid.
- Although members of a given species all have the same genes in their genome, individual
members of a sexually reproducing population may, by random mutation, have different versions
of the same gene, called alleles. Diploid organisms have two alleles for each gene in the genome,
one from the male parent and one from the female parent. Different alleles of the same gene can
give rise to different morphological or biochemical traits, or phenotypes. In some cases, one allele
tends to mask the phenotype of another allele. The former is said to be dominant and the latter
is said to be recessive. - In genetics, the Law of Segregation refers to the fact that, during meiosis, the two paren-
tal alleles of each gene segregate (i.e., separate from each other) so that the gamete (or in the
case of plants, the gametophyte) receives only one of the two alleles. The Law of Independent
Assortment states that when multiple traits (such as purple flowers, wrinkled seeds, and dwarf-
ism) are inherited, the genes controlling these traits assort independently from one another dur-
ing meiosis. Independent assortment allows traits to form new combinations with other traits,
giving rise to novel phenotypes.