The giant saguaro cactus as a living environment, akin to a deciduous tree--in a deft adaptation, for young children, of the stock of cactus-lore in Holmes' distinguished Cactus: The All-American Plant (1982). To the accompaniment of Lerner's microscopically detailed yet atmospheric illustrations, the cactus seed sprouts, a tiny green stem appears (""covered with little spines to protect it"") and starts to grow--""very slowly."" ""After one year the stem was no bigger than a kernel of corn. After five years it was only one inch high. . . At age 55 the cactus had reached 15 feet. Then it grew its first branch."" To the full-grown cactus come a pair of Gila woodpeckers, looking for a nesting place; ""They get sticky and wet as they work building a narrow entrance and a deep room about the size of a small football."" (None of us will ever see what the cutaway-illustration shows: three eggs in a deep hollow inside the cactus.) When the baby woodpeckers hatch, the adults fred insects under the saguaro's spines and inside the buds on top of the stems. Other birds sip nectar from the giant cactus flowers and many animals eat the sweet red fruit. Meanwhile a screech owl comes to lay eggs in the woodpecker's empty nest. Individualized, precisely informative, altogether interesting--and invitingly pictured, page-by-largo-page.