083-130_Botany 11/8/04 11:58 AM Page 97
Adaptations for Protection
predatory animals are especially favored by natural selection and, conversely, the most vulnerable run a greater risk of extinction. Anyone who has carelessly brushed against a rose bush, cactus, or hawthorn tree, knows how well some plants defend themselves. The protective structures these species bear are classified by botanists into four categories, each an adaptation of a familiar plant part. Thorns are modified short branches grown from axillary buds and terminating in sharp, hard points; among other species, hawthorn (Crataegus spp.) and blackthorn (Prunus spinosa) are so Cactus leaves evolved into protective equipped. spines, the stem being the organ of photoSpines are modified leaves or synthesis. parts of leaves, such as projections from the margins of blades. For example, some cactus spines are evolutionary remnants of rigid petioles and midribs, well sharpened for protective purposes. In the absence of leaf
A thorn is a modiďŹ ed short branch. Note that this pyracantha thorn has grown from an axillary bud and, at least temporarily, bears a few leaves.
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