Flora Unveiled

488 i Flora Unveiled

it penetrate the embryo sac to the egg. He was also unaware of the existence of sperm cells
inside the pollen tubes. He thus described the fertilization process as the diffusion of an
activating fluid through the cell wall of the pollen tube tip to the embryo sac.
Hofmeister was also unaware of the specifics of pollen grain development. Many of the
details were supplied some thirty- seven years later by the remarkable Polish- German bota-
nist, Eduard Strasburger. In 1884, Strasburger described the generative cell and vegetative cell
of pollen grains, as well as the formation of the two sperm cells from the generative cell. The
generative cell is the cell that divides to produce the two sperm cells. During pollen grain
development, the smaller generative cell is engulfed by the vegetative cell, which goes on to
produce the pollen tube. Strasburger was also the first to detect the entry of the pollen tube
into the embryo sac. He went on to characterize fertilization in angiosperms as the fusion
of the sperm cell nucleus with the egg cell nucleus.

The Discoveries of Meiosis and Double Fertilization

In 1888, Strasburger demonstrated that the two types of spore- producing cells of angiosperms
(now referred to as megaspore mother cells and microspore mother cells) undergo a special type of
cell division, called meiosis (reduction division), which reduces the number of chromosomes^16
of the daughter cells by half. The number of chromosomes per nucleus of a typical somatic cell
is characteristic of a given species: humans have 46 chromosomes; Australian daisies have 2;
roses have 14, alfalfa has 32, horsetails have 216; and the fern Ophioglossum reticulatum has
1,260. During mitotic cell division, each chromosome is replicated so that the daughter cells
have the same number of chromosomes as the cell from which they were derived.
Strasburger showed that during meiosis the daughter cells receive only half the number of
chromosomes in the nucleus of a typical somatic cell. Thus, the microspores and megaspores
that result from meiosis receive only half the number of chromosomes as the microspore
and megaspore mother cells that produced them. Since the male and female gametophytes
develop from their respective spores by simple mitotic (replicative) division, they have
the same number of chromosomes as the microspores and megaspores. If the number of
chromosomes in the sporophyte is 2N, the number of chromosomes in the gametophyte is
reduced during meiosis to 1N. (“N” in this case equals the number of chromosomes making
up the complete set of genes of a given species.)
During the process of syngamy, the nuclei of the egg and sperm cells fuse, providing the
zygote with two complete sets of chromosomes, one from the male gametophyte and the
other from the female gametophyte. In this way, the 2N number of chromosomes is restored
in the newly formed sporophyte. Cells with the 2N number of chromosomes are termed
diploid, whereas cells with the 1N number of chromosomes are termed haploid. Thus, the
sporophyte is made up of diploid cells, while the gametophyte is comprised of haploid cells.
Take, for example, a rose bush. The diploid number of chromosomes for a rose is 2N = 14.
All of the vegetative structures of a rose, including root, stem, leaves, and floral organs
(sepals, petals, stamens, pistils), have the diploid number of chromosomes (14). Therefore,
all of these structures comprise the sporophyte generation, which produces spores asexually.
What you see when you look at a blossoming rose bush is thus an asexual, spore- producing
plant, comparable to a mature fiddlehead fern.