Of the five kingdoms of living organisms, the three we have discussed so far consist of organisms that are all small relative to us. Bacteria and most protists are unicellular, and fungi, though multicellular are rarely as large as the individuals of many members of higher plants and animals. In the plant kingdom we encounter large individuals. It is not at all unusual for a tree to be more than 30 metres tall, with a mass of many metric tones. Plants are the dominant organisms of the terrestrial landscape, and they were the first organisms to invade the land successfully from the sea, where life first evolved and flourished. Algae and photosynthetic bacteria carry out most marine photosynthesis and plants are the dominant photosynthetic organisms on land. Plants are multicellular, eukaryotic organisms that have cellulose-rich cell walls, chloroplasts that contain chlorophylls a and b together with carotenoids, and, as their primary carbohydrate food reserve, starch.