For the lower vascular plants the important evolutionary development was in the water and food conducting tissues of the sporophyte. As we move on through the plant kingdom the next important development was the seed. The free living gametophyte is a vulnerable phase of the life cycle. Reproduction by seeds is a less chancy procedure and has other advantages for plant survival and dispersal. Seeds can be remarkably tolerant of environmental extremes heat, cold and drought. Unlike free-living gametophytes seeds can postpone their development until conditions are right. And, of course, we find them very convenient for plant propagation.1
Already in the coal-measure forests there were plants that reproduced by seeds. Some were the so-called "seed ferns". none of which survive. Others were the ancestors of the plants we now know collectively as "gymnosperms". In these plants the seeds are not enclosed in an ovary, as in the flowering plants; they grow on the surface of a modified leaf in a strobilus or cone. "Gymnosperm" means naked seed.1
Alternation of generations is still involved in the reproduction of these plants. They are all heterosporous: the microspores are shed as pollen, whereas the megaspore germinates in the strobilus to produce the female gametophyte. The archegonia in this gametophyte get fertilized by sperm from the male gametophyte and the zygote grows to produce an embryo which is enclosed in a seed coat of tissue from the parent plant.1
Gymnosperms were the dominant land plants in the age of dinosaurs, the Cretaceous and Jurassic periods. The surviving gymnosperms in the Coniferophyta, Cycadophyta and Ginkgophyta are similar in their woody habit and pattern of seed development but are not closely related.1
Some gymnosperms are among the tallest, largest and oldest organisms in the world. In New Zealand, rimu (Dacrydium cupressinum) and kahikatea (Dacrycarpus dacridioides) reach up to a height of 60m. Some of the tallest vascular plants in the world are the Californian redwoods (Sequoia sempervirens) that grow 117m tall. In the White Mountains of eastern California are the oldest living trees, the Bristlecone pine, Pinus longavera. One tree is estimated to be 4900 years old!3
New Zealand too has some very large and old trees. The best known of these are the kauri (Agathis australis) trees. One of our largest trees is 58m tall and has a circumference of 17.2m and is estimated to be over 2000 years old! This particular tree is popularly known as 'Tana Mahuta' or 'God of the Forest'. 3
Conifer leaves are needle or scale-like. They result from the downsizing of true megaphylls and unlike the microphylls of lower plants they are connected to the vascular system of the stem. Conifers are often large and can dominate the plant life in some ecosystems because their stems continue to expand in width as well as length throughout the life of the plant. The older parts of the stem become woody, which provides a...