Vines and Climbing Plants of Puerto Rico and the Virgin Islands

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8 Vines and Climbing Plants of Puerto Rico and the Virgin Islands Fig. 1. Stem cross sections. A. Pisonia aculeata. B. Ipomoea violacea. C. Pinzona coriacea. D. Chamissoa altissima. E. Securidaca virgata. F. Hyperbaena domingensis.
Vines and Climbing Plants of Puerto Rico and the Virgin Islands 9 the total diameter of their stems. These stems that are specialized for the conduction of water are known only in plants that possess xylem vessels (elements with perforated walls), and are absent in those that have only tracheids or imperforate elements. Imperforate elements obviously represent an obstacle to the free flow of water, slowing it down and making water transport over great distances difficult. The stems of climbing plants face structural challenges that differ from those experienced by trees and shrubs. They are subjected to tensile and compacting forces, due to the movement of the structures (usually trees) that support them. For this reason their stem construction, with an alternation of vascular and parenchymatous tissues, gives them considerable flexibility to withstand these types of pressure. In addition, the stems of climbing plants are subject to friction against the host trees that can lacerate their bark and thus wound the phloem tissue. Many lianas have encountered a solution to this problem by having phloem tissue inside the xylem (Dobbins and Fisher, 1986). The arrangement of phloem tissue in relation to the xylem can produce patterns sometimes considered anomalous, which serve to characterize families or genera of lianas (Schenk, 1893; Carlquist, 1991; Gentry, 1991; Acevedo- Rodríguez, 1993). Due to their taxonomic utility, these patterns form part of the species descriptions presented in this book. For the purposes of this chapter, a brief summary is presented below to characterize in general terms the most common patterns that are known for the stems of the lianas of Puerto Rico and the Virgin Islands. These are described based on cross sections of mature stems, without taking developmental processes into consideration. 1. Discrete vascular bundles. This pattern is the result of the activity of successive bands of cambium, which produce discrete bundles of xylem and phloem surrounded by parenchyma cells. The resulting pattern is that of collateral bundles dispersed in connective tissue (parenchyma). Examples of this type of pattern are found in the genera Bougainvillea and Pisonia (Fig. 1A) of the family Nyctaginaceae. 2. Alternation of bands of vascular tissue with connective tissue. Concentric bands. This pattern is the result of the activity of successive bands of cambium that produce a band of vascular tissue (xylem and phloem) accompanied by a band of connective tissue (parenchyma). The cambial activity is repeated to produce successive concentric bands of vascular tissue and connective tissue. The connective tissue can be as wide as the vascular tissue, thus producing a conspicuous pattern of alternating bands. This pattern can be observed in members of the genus Ipomoea (Fig. 1B) of the family Convolvulaceae and in the genera Pinzona (Fig. 1C) and Doliocarpus of the family Dilleniaceae. On the other hand, the band of connective tissue can be very narrow and hardly discernible, giving the appearance of xylem made up of successive concentric layers. Examples of this pattern can be observed in the genera Chamissoa (Fig. 1D) and Pfaffia of the family Amaranthaceae. Non-concentric bands. This pattern, like the previous one, is the result of the activity of successive bands of cambium. In this case, however, the activity of the cambial tissue gives rise to asymmetric bands, which develop primarily toward only one sector of the stem, thus producing a stem whose pith is not in a central position. Examples of this pattern are seen in the genus Securidaca (Fig. 1E) of the family Polygalaceae, in the genus Machaerium of the family Fabaceae, and in the genus Hyperbaena (Fig. 1F) of the family Menispermaceae. 3. Polystelic stems. A. These can be produced by stems with supernumerary cambial tissue whose secondary growth forms a stem composed of numerous vascular cylinders. In this pattern the peripheral vascular cylinders are of a smaller diameter than the central cylinder and can be seen both in young stems and in mature ones. Examples of this pattern are found in the genera Paullinia (Fig. 2A) and Serjania (Fig. 2B) of the family Sapindaceae. B. These are also produced by the successive production of cambial tissue within the cortical tissue, which develops into peripheral vascular
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Acelga trepadora ..................
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Wild cucumber .....................
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