If the leaves of all plants could function normally under any environmental condition, various leaf modifications would provide no special benefits to a plant. But the form and structure of tropical rain-forest plants do not adapt them to thrive in a desert, and cacti soon die if planted in a creek because their structure, form, and life cycles are attuned to specific combinations of environmental factors, such as temperature, humidity, light, water, and soil conditions. The modifications of leaves occupying any single ecological niche may be very diverse, resulting in such a rich variety of leaf forms and specializations throughout the plant kingdom that only a few may be mentioned here.
A single tree may have leaves that superficially all appear similar, but close inspection may reveal various differences. For example, because leaves in the shade receive
Figure 7.11 Portions of cross sections of maple (Acer) leaves. The chloroplasts are stained red. A. A leaf exposed to full sun. B. A leaf exposed to shade. Note the reduction in mesophyll cells and chloroplasts in the shade leaf.
upper epidermis palisade mesophyll spongy mesophyll lower epidermis
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