. A textbook of botany for colleges and universities ... Botany. LEAVES 641 In some instances petioles are of value in protecting developing buds from trans- piration and other detrimental factors, as in Acer, Platanus, and Rhus, where the buds for the next season are hidden more or less completely under the base of the petiole until leaf fall, by which time the bud scales are fully formed (figs. 940, 941). In various developing umbellifer shoots (as in Angelica, fig. 942) the petioles have large sheathing bases which enclose all the younger parts. In some aquatic plants the petioles are great
. A textbook of botany for colleges and universities ... Botany. LEAVES 641 In some instances petioles are of value in protecting developing buds from trans- piration and other detrimental factors, as in Acer, Platanus, and Rhus, where the buds for the next season are hidden more or less completely under the base of the petiole until leaf fall, by which time the bud scales are fully formed (figs. 940, 941). In various developing umbellifer shoots (as in Angelica, fig. 942) the petioles have large sheathing bases which enclose all the younger parts. In some aquatic plants the petioles are greatly inflated, air-containing organs that help to float the inflorescence (as in Utricularia inflata) or the entire plant (as in Eichhornia). Variations in the length of petioles, due to external factors, will be considered under stems (p. 728). Stipules. — Many plants, especially dicotyls, possess leaf appendages known as stipules, which usually occur in pairs, one at each side of the petiole near its Figs. 942-944. — 942, a growing shoot oi Angelica airopurpurea, showing the large inflated petiole (p) of a full-grown leaf, from which a young leaf (/) is just emerging; the petiole (J>') of this young leaf is cut away enough to show within it a still younger leaf (/'); the petiole (/>") of this younger leaf contains within it a still smaller and younger leaf ; 943, a part of a young shoot of a wild pea (Lathyrus ochroleucus), showing the prominent stipules {s, s'), and a young leaf (/) terminated by a tendril (/); the stipules early develop to their full size, for a time being erect and enclosing an undeveloped shoot, as at s'; 944, a growing shoot of a loosestrife (Lysimachia), showing a gradual transition from the early scale leaves {s) to the foliage leaves (/) that appear later; note also the decussate phyllotaxy. Their presence or absence apparently is unrelated to external factors and often charac- terizes entire genera or families; the Rosaceae and th
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
