An important reason for this difference is the sedentary lifestyle of plants. While some animals are sedentary, such as barnacles and sponges, most are quite able to move around. Thus, when conditions become uncomfortable, they may simply move away to another location. By contrast, plants are generally unable to move to a new location, but must rather cope with the circumstances in which they find themselves. For example, if a lion began to nibble on your leg, you would probably try to run away, however when a grasshopper begins to nibble on the leaf of a plant, the plant cannot simply run away. Instead plants must deal with the situation where they stand, by such means as producing noxious chemicals, growing spines, and generating sticky saps to deal with this problem.
Plants must also deal with heat and water in the place where they grow. If you become hot, you would simply move into the shade; if you are thirsty, you go for a drink. Plants do not have these options. In order to deal with heat, plants use water to soak up much of the heat, and then allow it to pass out of the plant as water vapor. Other plants will produce thick cuticle or reflective hairs to reduce the amount of light and heat they receive.
Water, or loss of it, can be a serious problem as well, and land plants have developed two basic strategies for coping. The first is dessication tolerance, and is most common among liverworts and mosses. These plants produce compounds which protect their cells when they dry out, and also have repair mechanisms which rapidly go into effect when the plant rehydrates. This strategy allows such plants to completely dry out for long periods, and then to rapidly recover. Some mosses for instance, have been packaged in envelopes in collections for several decades, and then brought out, rehydrated, and allowed to begin life again. Some mosses actually recover fully in under an hour. The other strategy for dealing with water loss is drought avoidance, in which plants develop features which reduce the loss of water, or store it in their tissues. This is the strategy taken by vascular plants, which have special vascular tissues to distribute water throughout their system. Some vascular plants have also developed succulence, a condition in which the tissues are spongy and swollen for storing water, as in cacti and agaves.
