dS2 CLIMBING PLANTS. 



Subsequently, the first, lower prop has perished, while the branches supporting 

 the upper portion of the liana remain still vigorous and afibrd a good hold. An 

 erect corkscrew-like liane stem is then seen hanging from the upper branches; 

 it has a very odd appearance, and is only surpassed in the peculiarity of its 

 form by the twined stems of bauhinias and monkey ladders, to be described 

 presently. 



But if the erect, young stem is stronger and more vigorous than the twiner 

 which encircles it, which has been used as a prop, it does not allow itself to be 

 strangled; the twiner is destroyed when they both increase in thickness. The 

 coils of the chmber are gradually stretched tighter and tighter, and many 

 are the contrivances which exist for preventing the tension from immediately 

 acting injuriously on the movement of the sap in the interior of the twining 

 liane stem. As this thickening continues, the pull on the coils becomes so 

 great that the death of the liane results. With its decay, the coils of the liane 

 offer no further resistance to the enlargement of the stem within; but become 

 ruptured and unravelled. It is clear from this that it is not always advantageous 

 for perennial and lignifying twining stems to make use of active stems as sup- 

 ports, and it is also obvious why old and very thick tree-trunks are never seen — 

 even in tropical forests — -encircled by twining stems. But those growths whose 

 twining stems persist only through a single summer, and either perish entirely 

 after the ripening of seed, like those of the twining Polygonum {Polygonum, 

 Convolvulus), or else die down to the ground like those of the Hop (Humulus 

 Lupulus), would suffer no injury even if they were to twine round thick tree 

 trunks. Such plants which have to develop stems and leaves in the course of 

 a short summer, and to manufacture by the help of their green foliage the 

 materials necessary for the formation of flowers and fruit, must spring up from 

 the soil to the sunny heights as quickly as possible and by the most direct path. 

 This they can best do by using a thin thread as a support, certainly not by 

 twining round a thick tree-trunk. The path round a thick trunk would be much 

 too long; the material necessary for the building up of such lengthy coils would 

 be needlessly expended, and such a waste would be entirely opposed to the 

 economy of plant-life. Of covirae this does not imply that twining plants have 

 the capacity of seeking out the most suitable supports, or of selecting the most 

 desirable from amongst many. The capacity of selection is at all times only 

 apparent, and if hop stems never twine round props of more than 10 cm. diameter, 

 it is not because the hop shoots are able to recognize beforehand the unsuitability 

 of large coils, but because with such extensive spirals they lose the power of 

 firmly adhering to the stem. And with this we come to the description of the 

 processes of adhesion and torsion of stems, so far as they are accessible to ob- 

 servation. 



Like interweaving and lattice-forming stems, twining stems at first grow 

 directly upwards. The lowest internodes still remain erect whatever may be 

 the fate of those developing above them. After a sufEcient number of successive 



