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 forms of the treetops of moist regions 

 penetrate farther into dry regions than do other epiphytes. Even in 

 northern forests a similar stratification exists, xerophytic lichens occur- 

 ring in the branches, while farther down are mosses and liverworts. 



see much advantage in the changed habit, since the new forms are quite as erect and 

 strong as ordinary individuals, and coils often occur in positions where they scarcely can 

 be of use, as at the base of a shoot. 



Fig. 968. — A pen- 

 dulous stem of the Vir- 

 ginia creeper (Psedera 

 quinquefolia)y showing 

 the curvature of lat- 

 eral branches upward 

 toward the light; note 

 the conspicuous leaf 

 scars (r) of the previous 

 season. 



