used to coat the entire plant in the move onto land. But the evolution of the cuticle, which protected the liquid-filled plant cells inside, created a new problem: it cut off ready access to carbon dioxide. In the ocean, carbon in dissolved carbon dioxide was simply absorbed across the cell wall. So to accomplish this in the newly evolved land plant, many small holes, called stomata, evolved as tiny portals for the entry of gaseous carbon dioxide.

The plant body must be anchored in place, and early land plants were probably anchored by fungal symbionts. Additionally, the symbiotic relationship between plants and fungi would provide for a means through which water could be recovered from the soil.

Moving onto land also created the problem of support. Plants need large surface areas facing sunlight so that their chloroplast receive enough energy through light to run the photosynthetic reactions necessary for plant life. One solution is to simply lie flat on the ground, and the very first land plants probably did this. This kind of solution is still used by mosses, which grow as flat carpets lying over soil. A visit to the Ordovician land probably would have been a visit to a moss world, where the world’s tallest “tree” was all of a quarter-inch tall. But this is a very limiting solution. Growing upright enables the acquisition of much more light, especially in an ecosystem where there is competition between numerous low-growing plants, and harder material was incorporated by early plants to allow first stems and finally tree trunks. Concomitant with the evolution of stems would have been the evolution of a transport system from the newly evolved roots up to the newly evolved leaves. Finally, reproductive bodies that could withstand periods of desiccation evolved, enabling reproduction in the terrestrial environment. With these innovations the colonization of land by plants was ensured, and with the formation of vast new amounts of organic carbon on land for the first time, animals were quick to follow. New resources spur new evolution.

As with plants, a major problem facing any would-be terrestrial animal colonist is water loss. All living cells require liquid within them, and living in water does not create any sort of desiccation problem. But living on land requires a tough coat to hold water in. The problem is that solutions that allow a reduction in surface desiccation are an-



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