In the tropical parts of Mexico and Central America, sennas are graceful trees with luxuriantly leafy canopies and brilliant yellow flower clusters. In adapting to the arid deserts of southeastern California, spiny senna, a two-foot-tali shrub, has kept the yellow flowers and discarded the rest. Since water is a scarce commodity in deserts, many desert plants, spiny senna among them, produce only ephemeral leaves. They rely on their green stems to do the work of photosynthesis, producing the sugars necessary for growth.
In the spring, the bare stems put out flowering shoots adorned with pretty yellow flowers and sparse leaves. Each leaf is composed of four to six paired leaflets that drop within a month or two. Afterwards, the leaf axis remains as a feeble spine. Spininess is common among desert plants; no one is quite sure why. Defense against herbivores is one suggestion. Anothei is that spininess is a consequence of the arid environment. What is certain is that desert plants arrive at spininess by several different routes. Some, like catclaw and white thorn, develop spines by modification of tiny leaf bracts called stipules. Others, like spiny senna, press the leaf stalk or the leaf axis into service.
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