In this “Article of the Week” I want to talk about the generation of the gymnosperms and the angiosperms and want to give you a general overview about the reproductive organs of plants and how pollination exactly works,
I. General Overview
The most higher species, animal or plant, pass through two different stages of lives: a Haploid and Diploid Stage. In the most cases, living things exist in the diploid stage; that means, they have a duplicated set of chromosomes (e. g. a normal human has 2 x 23 chromosomes). If an individual has more than a duplicated set, we called it Polyploidy.
So, before reproduction, the organism begins to transform its duplicated set of chromosomes into a single set. This process is called Meiosis. During the Meiosis new cells will created but the total number of the chromosomes will not rises. So at the end of the Meiosis, we normally have many “male” and “female” cells with a haploid set of chromosomes.
For example, the sperm cells (male) or the ovum (female) of the animals are haploid cells. Plant will even made an entire, small haploid organisms, which are called Prothallium. In many cases, especially at the higher Plants, the male Prothallia are much smaller than the females and have also different anatomy. In this cases, the male Prothallium is called Microprothallium and the female Makroprothallium.
At the high plants, the female Makroprothallium.is also known as Ovule and the male Microprothallium as Pollen.
II. Anatomy of the ovule
The ovule is the result of many cell divisions. It starts after the Meiosis with a haploid cell: the Mother cell. This cell germinates and is divided into four smaller daughter cells. However, from these four cells only one will survive. This cell will multiply its chromosomes, so in the end, we have a oktoploid (angiosperms) or heptaploid (gymnosperms) cell; the pre-form of the ovule. Later, this pre-form is subdivided into seven smaller cells again; this is finally our ripe ovule.
So the ovule consist of seven different cells. The biggest one is the embryo sack. This diploid cell is later responsible for the nutrition of the embryo. The angiosperms have a diploid embryo sack, the gymnosperms a haploid one (this is a basic difference between the gymno- And angiosperms).
Then, we have the haploid ovum, which is flanked by two helper cells, the synergids. On the opposite side, there are also three other helper cells, which are called antipods.
The ovule is surrounded by a endosperm, which is called the Nucellus. Ovule and Nucellus are protected by one (gymnosperms) or two (angiosperms) layers: the integuments.
At the base, the integuments are often fused. At the top, the integuments are forming a small tube, which is the entrance to the ovule. This entrance is called Micropyle; the fused area is called Chalaza.
Finally, the whole ovule is located at the end of a short stalk: the funicle, which connects the ovule with the placenta.
At the angiosperms, the ovule is enclosed by the ovary, that is formed by the carpels. At the gymnosperms the ovule is free and protected by the seed scales.
III. Anatomy of the pollen
The anatomy of a pollen is more simply than the anatomy of the ovule. It consists of two haploid cells: the “pollen tube mother cell” and the “spermatozoon mother cell”. The first one is responsible for the growing of the pollen tube, the second one for fertilization. They are both enclosed by a callose layer. The surface of this layer can be shaped very differently. For example, some pollens has air filled wings or a very spiky surface.
After the polen has landed at the stigma, the “pollen tube mother cell” breaks through the callose layer and grows through stylus and ovary right to the ovule. Here it enters the ovule by the micropyle and grows in the synergids.
After the pollen tube is completed, the spermatozoon mother cell divides itself into two small spermatozoon cells. This cells wander through the pollen tube in the ovule. Here, one spermatozoon enters the nucellus and fuses with its two cores, forming a triploid tissue that is called the secondary endosperm. The other cell enters the ovum and its haploid core fuses with the core of the ovum, what is the fertilization. After successful fertilization, the now diploid ovum starts to grow and becomes the plant embryo. During the first hours of it life, the embryo is feed by the nucellus, later by the secondary endosperm.
The pollination of the gymnosperms is very similar to this process, but here, the pollen is caught by a special drop, that grows through the micropyle. This drop is called the pollination drop. Another different is, that the gymnosperms have no double fertilization. Only the ovum is fertilized by on spermatozoon, the endosperm remains haploid.