This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The scales on the bulb of a Lily are modified by being fleshy, without chlorophyll, and filled with starch. Those of the Daffodil and Onion are made up of the sheathing bases of leaves, and go right round the bulb. The Onion stores a glucose-like substance or grape sugar in the sheaths of the bulb. The stored materials enable the plants to make a good start into growth the following year, and to throw up their flower stems. In late summer and autumn the apex of each shoot and twig of most trees is covered with brown scale-like leaves for the purpose of protection. The flowers of the Peach, Horse-chestnut, and Rhododendron are covered in the same way during the winter. The scales are simply ordinary leaves arrested at an early stage of growth, and have the same arrangement as they. At the base of the individual flower stalks in many plants are small leaves, termed bracts; in the Carnation they come close up to the base of the calyx in two pairs. The bracts are very numerous in the Marguerite, Cineraria, and other composites, and closely surround each flower head, like overlapping scales. The bracts surrounding the clusters of flowers of the Poinsettia are large and highly coloured.
The leaves of some plants, like the Sarracenia (fig. 41), Darlingtonia (fig. 41), Nepenthes or Pitcher Plant (fig. 41), and Heliamphora nutans undergo complete changes in appearance for special purposes, chiefly for the purposes of catching insects and afterwards digesting and absorbing them. In the case of the Sarracenia, Darlingtonia, and Heliamphora the leaves roll themselves into a tube or trumpet, on the inner surface of which a honey-like substance is secreted, and on which numerous sharp, bayonet-like hairs point downwards. The insects, being attracted by the honey, enter and feed till satisfied, but when they attempt to get out they are repulsed by the bayonet-like hairs, and eventually sink back exhausted and die.
In the case of the Pitcher Plant (Nepenthes) the leaves are normal flat expansions for a considerable length. The tissue on each side of the midrib suddenly ceases to grow, and the midrib develops by itself for several inches. Eventually, however, a pouch-like organ or "pitcher" is formed at the tip, and is provided with a lid. This soon opens and allows the ingress of various insects, which ultimately meet the same fate as those entering the other plants referred to. In fig. 41 (4) the stout downward-pointing teeth around the rim of the pitcher are shown.
Fig. 41. - Pitcher Plants.
1, Sarracenia variolaris. 2, Darlingtonia californica. 3, Sarracenia laciniata. 4, Nepenthes villosa, reduced to one-half natural size.
Another plant with similar contrivances of modified leaves is the Australian Pitcher Plant (Cepha-lotus follicularis), (fig. 42).