. A textbook of botany for colleges and universities ... Botany. STEMS 657 PendMous plants. — Various vines, as the Virginia creeper, if rooted at the top of a narrow caiion, may hang down over the wall; however, such vines are prophoto- tropic and apogeolropic, as may be seen by the recurved tips, and by the petioles which point upward toward the light (fig. 968). Leaners sometimes behave similarly, the most remarkable feature being that the pendulous stems are several times as long as are ere'ct stems in the same species (as in Rubus occidentalis and Ribes Cynosbati). The cause of this elong
. A textbook of botany for colleges and universities ... Botany. STEMS 657 PendMous plants. — Various vines, as the Virginia creeper, if rooted at the top of a narrow caiion, may hang down over the wall; however, such vines are prophoto- tropic and apogeolropic, as may be seen by the recurved tips, and by the petioles which point upward toward the light (fig. 968). Leaners sometimes behave similarly, the most remarkable feature being that the pendulous stems are several times as long as are ere'ct stems in the same species (as in Rubus occidentalis and Ribes Cynosbati). The cause of this elongation is unknown, though it seems possible that it is in some way associated with the fact that growth is in the direction of the gravity pull, instead of against it, as 'n erect stems. It has been found that when growing stems are subjected to tension, cell elongation takes place in the direction of the pull. Epiphjrtes. — General remarks. — Epiphytes are nutritively independent plants, which are given complete mechanical support by other plants, dif- fering from parasites in not deriving food or water from the supporting plant, and from lianas in having no soil connections. Although all gradations exist between lianas, epiphytes, and ordinary soil plants (as in Nephrolepis, p. 653), the most representative epiphytic forms occur only as epiphytes. In regions with winters or with prolonged dry periods, true epiphytes are limited essentially to algae, lichens, liverworts, and mosses; in the moist tropics, these forms are supplemented by many ferns and seed plants, especially orchids and bromelias (fig. 969). In many tropical forests the epiphytes are arranged in stories or strata; those in the treetops (such as species of Tillandsia) are very xerophytic in struc- ture, while farther down are more mesophytic forms, such as the orchids and ferns. Near the ground, where desiccation rarely takes place, there occur extreme mesophytes, such as the filmy ferns. The xerophytic for
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