The ANGIOSFERMS, which are much the larger class, derive their name from the fact that the carpel or carpels form a closed chamber, the ovary, in which the ovules are developedassociated with this is the development of a receptive or stigmatic surface on which the pollen grain is deposited.
The pollen grain bears numerous spines, the dark spots indicate thin places in the outer wall.
When placed on the stigma, under favourable circumstances, the pollen-grain puts forth a pollen-tube which grows down the tissue of the style to the ovary, and makes its way along the placenta, guided by projections or hairs, to the mouth of an ovule.
Before following the growth of the pollen-grain after pollination, we will briefly describe the structure of a cycadean ovule.
The number of chromosomes in the nucleus of the two spores, pollen-grain and embryo-sac, is only half the number found in an ordinary vegetative nucleus.
