The Primitive Garden, located next to Jenkins Hall on the Armstrong campus, presents a walk through time by displaying living relatives of ancient plants. Plants in the collection represent three important stages in the expansion of the earth’s flora. The invasion of land, along with the development of the seed and the appearance of flowering plants, represent the three most significant events in the history of plants.
The Invasion of Land
The first plants had it easy. They floated around supported by ocean, absorbed nourishment through their cell walls, and reproduced easily by releasing their reproductive cells in the water to meet and form tough capsules that could float around and find new places to live. Living on land was much tougher. Drying winds and large temperature swings prevented those early water plants cast ashore by waves or trapped in evaporating pools from living very long. To adapt to life on land, plants needed to develop substances to prevent their exterior surfaces from drying out, specialized cells for the transport of water and nutrients, and maybe most importantly, an effective method of sexual reproduction.
Early land plants found that effective method of reproduction utilizing spores produced by a unique process called alternation of generations. Alternation of generations is a cycle between adult plants and another stage that is kind of an intermediate phase. The intermediate phase is not the same thing as a juvenile stage. Juvenile plants eventually become adult plants while those in an intermediate stage will never become adult plants. The stages can be thought of as different generations within the same life cycle and are easy to see in plants that reproduce by spores. The adult generation, what we usually see when we look at plants, produces the spores. The spores germinate and develop into the intermediate generation. The intermediate generation produces sex cells- sperm and eggs. Union of these sex cells produces an adult plant and the cycle continues. The greatest limitation of this system, at least as it applies to early land plants like ferns and horsetails, is the necessity of a film of surface water for the sperm to swim in to unite with the eggs. No free water means no reproduction and explains why ferns don’t usually grow and reproduce in arid environments. Within the plant kingdom, dominance of generations varies. The earliest land plants, like mosses and lichens, have the intermediate generation dominant, so when you see moss growing on the ground you are looking at the intermediate generation of its life. Ferns and horsetail produce small but completely independent intermediate generations. Through time, plants show a progression of increasing adult generation dominance and intermediate generation reduction from the ferns up to the flowering plants.
The Development of the Seed
Most early land plants produced one size of spore. Upon germination, these spores grew into the intermediate generation which produced both male and female sex cell producing organs. In many of these plants it is possible for sperm to fertilize the eggs from the same intermediate plant. This causes problems when the success of a species relies on genetic variability in a population.
Some early spore producing plants produced two different sized spores; a larger spore that produced female sex cells, and a smaller one that produced male sex cells. At some point in time, the adult generation no longer released the female spore to develop into an independent, intermediate generation. Instead, the female spore developed into an egg producing intermediate generation on the adult and became completely dependent on it. Male sex cells in the form of pollen were transported to the female eggs by the wind, thus freeing plants from their dependence on surface water to reproduce. Larger female spores contained a greater food reserve ensuring nourishment for the undeveloped, adult plant in the form of a seed. Seeds are dormant, protected, embryo adult plants. The role of dispersal once assumed by spores was taken over by the seed.
The development of the seed provided a major innovation in plant reproductive strategy and is certainly the primary reason for the dominance of seed plants in today’s flora. In examining the seedless plants, it becomes obvious that the adult generation can only grow where the intermediate generation can succeed. Because of its dependence on water, the intermediate is the weak link in the life cycle. The development of the seed allowed plants to reproduce sexually without the necessity of external water, and provided protection and nutrients for the developing embryo. Seeds can be dispersed long distances and can remain viable for long periods of time. (How long can seeds last? Carbon-14 dating has shown that 10,000-year-old seeds of Lotus are viable and will still germinate!) These traits allowed plants to expand their range away from streams and watery sites to higher and drier environments.
Most early seed plants relied on wind to transfer pollen from plant to plant. Large quantities of pollen must be produced to ensure that some will reach its target. Wind pollination is most effective in plant communities where the wind-pollinated plants are dominant, common, and closely spaced. Coniferous forests and grasslands represent good examples of wind pollinated plant communities. Botanist theorize that some of the early wind pollinated plants produce resins and gums on their egg producing structures to increase the chances of pollen sticking to them. These gummy substances also possibly attracted insects interested in a sticky treat. The sticky insects unknowingly transferred pollen stuck to their bodies from one plant to the next and insect pollination was born. Plants continued to develop increasingly attractive structures and substances, like flowers, nectar, and fruit to increase the likelihood of animals assisting them in pollination and dissemination. Flowering plants took over most terrestrial habitats in an astonishingly short period of time displacing many ferns and non-flowering seed plants. How did flowering plants and their animal helpers so thoroughly take over most of the earth’s plant communities? Imagine a large coniferous forest made up of pines and spruces. A flowering plant on the fringes of this forest with help from an animal seed disperser could move into the under story and grow in an opening left by a tree fall. A wind pollinated plant would have little chance of being pollinated being so isolated from others of its species but the flowering plant with its mobile animal pollen vectors could be successfully pollinated. Structural differences also allow flowering plants to grow more rapidly than the non-flowering seed plants so our isolated plant in the forest quickly out competes any of the existing plants around it for sunlight and water. Animal pollination allows plants to be smaller, more isolated, and less common than wind pollinated plants. Animal pollinators will travel considerable distances from plant to plant to seek the reward of a “sticky treat”.
Last updated: 6/26/2018