what does the tattoo artist mean by "exchange"? how does one trade a name for a number in devil's arithmetic

[1] nothing that comes from the desert expresses its extremes better than the unhappy growth of the tree yuccas. tormented, thin forests of it stalk drearily in the high mesas, particularly in that triangular slip that fans out eastward from the meeting of the sierras and coastwise hills. the yucca bristles with bayonet-pointed leaves, dull green, growing shaggy with age like an old [5] man's tangled gray beard, tipped with panicles of foul, greenish blooms. after its death, which is slow, the ghostly hollow network of its woody skeleton, with hardly power to rot, makes even the moonlight fearful. but it isn't always this way. before the yucca has come to flower, while yet its bloom is a luxurious, creamy, cone-shaped bud of the size of a small cabbage, full of sugary sap. the indians twist it deftly out of its fence of daggers and roast the prize for their [10] own delectation why does the author use the words "bayonet-pointed" (line 4) and "fence of daggers" (line 9) to describe the leaves of the yucca tree? . to create an image of the sharp edges of the plant to emphasize how beautiful the plant's leaves are to explain when and where the plant grows to show how afraid the author is of the plant