654 



ECOLOGY 



severed through the decay of the posterior portions, thus resulting in the transforma- 

 tion of a soil plant into an epiphyte or an epiphytic liana, though the roots, whether 

 in the soil or in the bark, are true soil roots, which absorb water and salts from the 

 substratum (fig. 964). Such climbing rhizomes often keep pace with the ascent of 

 the supporting tree, and in the palmetto they are found most commonly in the 

 enlarged spongy region just below the leaf crown. The rhizome of Polypodium 

 aureum often creeps around palmetto trunks in a slowly 

 ascending spiral. Rhizome climbers do not always start 

 from the ground, since spores may germinate at any 

 level on the trunk. Outside of the tropics climbing by 

 rhizomes is illustrated by several species of Polypodium ; 

 various creeping mosses and liverworts ascend tree trunks 

 in similar fashion, especially in swamps, and embedded in 

 the mosses, there occasionally may be seen such rhizomes 

 as those of Maianthemum. 



Plants which climb by hooks or thorns. In hook climbers 

 and thorn climbers attachment or connection with a support 

 is in a sense accidental, involving no special growth features 

 as in the previous groups of lianas. Climbing by thorns is 

 seen in roses, in blackberries, and in the greenbrier; the last, 

 however, is supported much more effectively by tendrils. 

 Hooks that point downward characterize various bedstraws 

 (Galium) and the hop (fig. 965), and are superior to thorns 

 in that they prevent slipping backward. 



Leaner s. A transition between lianas and ordinary erect 

 plants is afforded by one of the night-shades, Solanum 

 Dulcamara, which leans on neighboring plants, having no 

 means for climbing, except that it sometimes twines to a 

 slight extent. Such a plant may be called a leaner. The 

 tall nasturtium (Tropaeolum) is another such leaner, which 

 also may climb to some extent by petiole twisting. Many 

 plants that are prostrate in the open may be leaners where 

 the vegetation is dense. Closely related to leaners are those 

 submersed aquatics whose stems are supported by the water, their mechanical 

 tissue being insufficient to keep the plants erect. 



FIG. 965. A dia- 

 grammatic longitudi- 

 nal section through the 

 outer portion of a hop 

 stem (Humulus Lupu- 

 lus), showing emer- 

 gences with obliquely 

 oriented barbs, the 

 lower ones (h) point- 

 ing downward and out- 

 ward in such a way as 

 to hold the stem to a 

 support; somewhat 

 magnified. 



The advantages and disadvantages of the liana habit. The great 

 advantage possessed by lianas is their favorable position for foliage dis- 

 play without the construction of a large amount of supporting tissue. 

 When a vine sprawls over a hawthorn, the liana may have a foliage 

 display equal to that of the tree. However, this habit may be accom- 

 panied by ultimate disadvantage, because the leaves of the liana cut 

 off the light from the leaves of the supporting tree ; also the weight of 

 the vine may become too great for the tree, resulting in the downfall 

 of both (fig. 966), The collapse of a branch may suffice for the down- 



