A big world in small powder
Which plants produce pollen?
Not all plants produce pollen, which is the male gametophyte of seed plants.
Plants are divided into higher plants and lower plants according to whether they have morphologically differentiated roots, stems, and leaves, and whether they have “embryonic” structures in their ontogeny. Lower plants include algae, lichens, etc.; Higher plants include bryophytes, ferns, and seed plants. Seed-plants, at the end of the evolutionary line in the present plant world, say that only the higher seed-plants of “us” higher plants can produce pollen. Mosses that grow in wet places, ferns that can easily attack you visually (some ferns have dense, neat piles of spores on the underside of their leaves), and so on, do not produce pollen. Their gametophytes are called spores.
The male (left) and female (right) bulbs of the iron tree
In addition, different seed plants produce pollen in different places. Seed plants are divided into angiosperms and gymnosperms. Among them, angiosperms flower and pollen is formed in the stamens; Gymnosperms do not have true flowers; pollen is produced by their reproductive organ, the male bulb.
Far away, in the late Devonian period, there were vast, shaded forests of lycophytes and eupen ferns. Around that time, early seed plants appeared somewhere on Earth, and pollen has been carrying DNA for more than 300 million years. Now, we can also get a glimpse of what pollen looked like back in time, based on ancient and rare fossils.
At right is the oldest fossil pollen ever found. It spent 243 million years in the ground and was squashed and deformed until it returned to the air in 2013. Scientists think it probably came from a angiosperm plant.
The oldest fossil pollen ever found
The pollen failed to pollinate that year and was not lucky enough to meet a flower in full bloom to share its DNA. Although the germ cells that pollen carries are long gone, the protective outer wall of pollen has been helping paleobotanists for more than 200 million years. Previously, scientists thought that the earliest angiosperms had only emerged during the Early Cretaceous (135 million years), and this suspected pollen fossil was 243 million years old. If the pollen is confirmed to have come from an angiosperm, it would push back the appearance of angiosperms by about 100 million years. 100 million years, so long, the earth revolves around the rotation of countless times, a small pollen calmly revealed to us the secrets of plant evolution.
Arabidopsis pollen and its outer wall
The outer layer of pollen grains is called the outer wall, which is why pollen fossils can be preserved in the ground for so long. The main component of pollen outer wall is spore pollenin, which is a complex polymer biopolymer with strong tolerance to acid, high temperature and pressure, and it is difficult to be oxidized even in the acidic state of water immersion. Today, scientists have access to fossil pollen in the ground to study it, because most pollen is resistant to corrosion.
Each pollen carries its genes through endless fields and mountains, or falls into puddles when the wind dies down, or is carried into a hive by a bee, or may be happily eaten by a bug. Only a few lucky ones land just enough on the waiting flower or female bulb to carry the germ cells to the right place. Despite the uncertainty about their future and whether they will be one of the lucky few, each pollen is meticulously developed into a delicate shape.
Pollen in all its shapes and forms
Plants of different families and genera have pollen of different appearance. These pollens vary in size and shape, and the palynologist (spore and pollen) can observe the morphology of the pollen through a microscope to determine which family or genus of plants the pollen came from. Of course, there are a few plants that can be accurately identified by pollen.
At the same scale, the differences in pollen size between different plants are starkly displayed. Forget-me-not pollen, for example, is less than 10 microns in diameter, compared to pumpkin pollen, which is larger than 100 microns in diameter.
Red pine pollen
Someone described the pollination of hazel trees as follows: “The hazel tree sighs and exhales pollen. When spring begins to warm, the male catkin of the hazel tree hangs down like a sheep’s tail with small flowers. When the wind blows, the inflorescences swing like small pompoms. A yellow cloud darkened the sky… Yellow clouds drift and rain pollen.” Hazel tree is wind-borne plant, its pollen surface is relatively smooth, easy to be blown away. The pollen of wind-borne plants is light and smooth, and some plants have two air sacs, like pinus pollen, that fly away when the wind blows. Some pollen can be blown thousands of meters away, but “the wind is really good, sending the powder into the clouds.”
Among seed plants, there are not only wind-pollinated plants, but also plants that pollinate by biological media such as animals and self-pollinating plants.
For angiosperms, the ability to attract insects and pollinate by producing flowers and fragrances is an innate skill for most species. And bees, ants and other small animals, attracted by pheromones or colors to approach flowers in search of food is also an instinct. The interactions between plants and pollinators are not fairy tales of warmth, but rather of intrigue. Fuzzy bumblebees, for example, may sometimes get tired of sticky powdery clumps on their bodies and nibble small holes in the outside of flowers to suck the nectar. This behavior of getting food while avoiding the responsibility of pollination is called “honey stealing”. Other flowers are happy to use radical methods to “force” insect pollination. For example, the flowers of the Lilia genus are very different from those of the female. The male flowers are very showy and the female flowers look like little green helmets. Once the orchid bee is attracted to the male flower, landing on the flower, accidentally touch the “organ”, the flower powder will be launched “bang”, hard stick in the orchid bee body. At this point, the frightened orchid bee scurries away from the seemingly “bad guy” male and into the small helmet-shaped female — thus, the orchid bee’s pollination is pitifully accomplished by the combination of the male and female flowers of the genus Flabblip.
Pollen has been observed under a microscope since the 1640s. In the early 20th century, scientists for the first time plotted the percentage variation of pollen in peatland based on the pollen composition of different layers.
As we mentioned earlier, the pollen shell is composed of spore pollens, which are stable. However, there are conditions for pollen preservation. Pollens scattered in the soil are easily digested by microorganisms, and pollens exposed to heat and humidity are easily oxidized. However, pollen trapped in peat or lacustrine sediments can be better preserved, sitting quietly in the ground for tens, hundreds of thousands, even millions of years. These ancient pollens are valuable material for paleoclimatologists.
Hoverflies gather nectar from flowers
Bumblebees are stealing honey
If you look at the plants along the way from Hainan Island to Mohe, it is obvious that plants grow very differently from place to place. The coconut tree in Hainan Island and the larch in northeast China are all plants that can only grow in the local climate. This law of the growth of different flora under different climatic conditions is the basis for paleoclimatologists to understand the climatic changes in different geological periods according to the changes of palynological composition in strata. Based on the percentage of pollen and statistical methods, paleoclimatologists can trace the temperature and precipitation changes over thousands of years.
Pollen, though small, is important in helping us understand the changes in the paleoclimate. Only with a better understanding of paleoclimate change can we deeply understand the mechanism of climate change and better predict future climate change.