Praised be my Lord for our sister, mother earth, The which sustains and keeps us And brings forth diverse fruits with grass and flowers bright.”
St. Francis of Assisi
Looking at the very thick atlas of the Kingdom of Plants (scientists have identified nearly 300,000 species!), we would find the regions (the divisions) split into two halves. In the first (and much smaller) half, you would have the divisions of algae, mosses, and liverworts, while in the second half you would have horsetails, ferns, conifers (like pine trees), gingko trees, and all the flowering plants. Why the two halves? Is the Kingdom of Plants in a civil war? The separation comes from a basic but important difference in the structure of these plants.
What is this difference? Consider moss on the one hand and a tree on the other: how are they similar and how are they different? They both grow out of a solid surface (usually the soil) and are green, but the moss stays very low to the ground while trees become very tall. This is because the tree is a vascular plant – it has features similar to tubes running through its roots, trunk, branches, and leaves that allow water, nutrients, and food to move through the plant. This transportation system allows these plants to grow much taller on stiff stems, as well as make it able to live in environments with harsher weather because it can more effectively get nutrients and store food as needed.
Mosses, algae, and horsetails are non-vascular plants – they lack this tube system and so must stay near the ground. The leaves on mosses and horsetails are extremely thin (often just one cell thick!) attached to a stem-like structure. They also do not have true roots, but, rather, hair-like structures (known as rhizoids) that help to anchor the moss and absorb a small amount of water, although moss mostly get their water from the air. Non-vascular plants also do not have seeds, but rather mostly reproduce using spores (similar to fungi), as we will talk about more later.
The vast majority of plants have a special feature that makes them some of the most stunning parts of creation: flowers. From daffodils to dogwoods, spring and summer are full of the smell and color of these creatures. Flowers are used to make seeds to grow more plants in the future. Except for non-vascular plants, nearly all other plants have seeds, many of which are tasty like walnuts and hazelnuts, or wrapped in a spiky shell like gum balls or pine cones, or sticky like burrs. Even corn, peas, wheat, and lentils are all seeds that we eat.
As we have with the other kingdoms, let us consider whether plants are autotrophs or heterotrophs. Recall that plants make their own food using the energy of the sun, which make them autotrophs. There are, however, a few species of plants that are carnivorous, meaning they eat animals. Plants such as the Venus flytrap and the pitcher plant live in soils with few nutrients, so they catch insects and other small animals to make up for what they lack from their environment. These plants do still photosynthesize, so despite consuming other creatures, these surprising plants are still autotrophs.
What about plant-like protists? The border between the Kingdom of Protists and the Kingdom of Plants often changes as scientists continue to consider which creatures are true plants and which are plant-like protists. To make matters more confusing, the creatures often have very similar names, such as green algae, red algae, and brown algae. Plants have cell walls that are made of a stiff carbohydrate known as cellulose; scientists generally consider creatures with chlorophyll and this kind of cell wall to be plants. Most plant-like protists, on the other hand, have cell walls made of mostly protein or silica (the same thing as sand); yellow-green algae even have cell walls made of some unknown material! Perhaps one day you will be the one to uncover the mysteries of yellow-green algae.