ECOLOGICAL 171 



discs borne by tendril-branches in some Virginia Creepers. In the 

 Japan ivy {Psedera tricuspidata) the tendril-branches end in little 

 knobs, which not only broaden out into discs when contact is effected, 

 but secrete a mucilage which makes fixation doubly sure. 



If a tendril fails to find any suitable support it dies, and similarly 

 the adhesive discs of Virginia Creeper {Ampelopsis veitchii) harden. 

 But Neger cites Czapek's description of a remarkable plasticity in 

 Entada polystachya, a strong liana of Tropical America. The leaves 

 bear branched tendrils, which twist and harden in the ordinary 

 way if contact with a support is effected. But if no support is reached 

 the tendrils develop leaflets, which help in photosynthesis. 



The chief advantage of the chmbing habit is that it enables the 

 chmber to spread out its leaves in the open light, and yet secures 

 this with an economy of supporting tissue. The rattan palm (Cala- 

 mus) may be over seven hundred feet in length, but its stem is 

 relatively slender. On the other hand, the disadvantage of the habit 

 is that the liana may smother or strangle or overweight its supporting 

 tree, with the result that both are brought fatally to the ground. 



In addition to marked mobility and sensitiveness, exaggerations 

 of qualities seen in some measure in all the higher plants, the 

 climbers show (i) flexibility and firmness in the axis (often associated 

 with a peculiar splitting up of the wood into numerous separate 

 strands, and a reduction of pith) ; (2) a lightness of build, so that 

 there is not overstrain on the actually attaching structures, such as 

 tendrils and hooks; (3) a proportionately high development of the 

 sap-conducting system, seen especially in an enlargement of the 

 calibre of the wood-vessels when the climber has reached a certain 

 height and thickness and has begun to lose much water from its 

 expanded foliage. 



As to the origin of the climbing habit, which occurs in so many 

 different orders and is effected in so many different ways, it is 

 reasonable to regard it as a specialisation of the nutation and sensi- 

 tiveness seen in most plants that have an axis and appendages. 

 The recent emergence of a twining variety of one of the snapdragons 

 {Antirrhinum majus) is suggestive of the possibilities that may lie 

 latent and unsuspected. It arose as a mutation, we cannot tell how; 

 and in addition to its incipient twining habit, it shows compacting 

 of wood and reduction of pith. Yet there is no recognisable advantage 

 in the new habit in this particular case, for the stem is strong enough 

 to stand on its own legs. The mutation breeds true, and shows that 

 a climbing race might arise as a germinal variation. 



Evolution. — Some light on the evolution of climbers in the wide 

 sense is afforded by experiments on the elongation of stems. Shade 

 and moisture tend to evoke prolonged growth of stems, as is 

 sometimes familiarly illustrated by potatoes sprouting in a damp 

 and almost dark cellar, or by strawberry runners spreading under 



