We saw in Chapter 4 (Box 4.1) that the large clubmosses produced two types of spores, large and small. The large ones developed ova-containing structures in which the ova were fertilized by spermatozoids released nearby from the microspores. The mobile spermatozoids required water from the environment in order to be able to ‘swim’ to the ova. From these primitive beginnings plants slowly developed a system of reproduction that would be entirely independent of the need for environmental water. On land the latter would not always be available at just the right time.
We do not always have the fossil evidence for all of the stages of the system but because a sufficient number of the types of primitive land plants succeeded in surviving to the present day, we can study their reproductive systems. We can arrange Cycas, Ginkgo and Pinus in a sequence which shows us how the stages probably went in fossil seed-plants. An introduction to this concept was given in Chapter 3 (Box 3.1). First of all there developed a place on the ovule, as the immature seed is called, where microspores could enter. This place is known as the micropyle which means ‘a small gate’. Once through the micropyle the microspores can remain, largely sheltered from the environment, in a cavity called the pollen chamber. Henceforth in this account we will use the term ‘pollen’ instead of microspore.
The entrapment of pollen at the micropyle is facilitated by a ‘pollination drop’ exuded through it by the ovule. Immediately we see here the production of fluid by the parent plant making it independent of environmental water. What happened next differs somewhat between Cycas and Ginkgo on the one hand and Pinus on the other. In the case of the first two plants a tubular outgrowth, the pollen tube, emerges from the pollen grain and grows towards the archegonium containing the ovum. It then releases two mobile spermatozoids which ‘swim’ towards the ovum and one fertilizes it. Pinus however appears to have evolved rather further than the other two plants and its pollen tube merely releases immobile nuclei in such a way that one of them can fertilize the ovum. Here the last remaining semblance of ‘fertilization in water’ is discarded so that the only thing remaining is the essential act of two nuclei fusing, one male and one female. After fertilization an embryo develops and the ovule matures into a seed ready for dispersal from the parent plant.
As has already been mentioned, the angiosperms, the flowering plants, developed a special container, the carpel, for their developing ovules. Although this undoubtedly proved to be very successful in protecting the young ovules, at the same time it erected a barrier between them and the pollen grains. This problem was overcome by the development on the carpel of a special landing place for the pollen grains so that they could then grow pollen tubes long enough to reach the ovules inside. This rather moist, sticky place on the carpel is known as the stigma and it is joined to the carpel by a stalk called the style. In the case of crocuses, it may be up to a few inches long but in most plants it is usually much shorter. We shall soon be meeting up with the first angiosperms in the next chapter.