4 THE BOOK OF CLIMBING PLANTS 



due to the pressure of circumstances and the desire of 

 nature to economise. The climber has generally to find 

 its way to the light and air by aid of the support of other 

 plants or surrounding objects of greater height, and had 

 it not its lengthened stem and its climbing powers it 

 would be strangled or destroyed by want of light and 

 by being unable to absorb the carbon dioxide it requires. 

 The elongation of the stem without additional thickness 

 would have been insufficient from its want of power to 

 sustain itself erect, and thus the powers of twining 

 round or clinging to something which would support it 

 were developed to aid in the struggle for existence. 

 These powers are provided in the shape of a twining 

 habit, tendrils, aerial roots, prickles, or hooks. Some 

 writers are disposed to separate twiners from other 

 plants with a climbing habit, but for gardening purposes 

 they ought to be dealt with together. The contriv- 

 ances by which these plants attain to the light and 

 support their stems are varied in their character, and, 

 though they might be divided broadly into a few classes, 

 yet there will in a few cases be found some which 

 possess a share of two or more means of climbing or 

 supporting themselves and cannot therefore be classed 

 with only one section. Yet they may generally be 

 divided into twiners, tendril-bearers and leaf-twisters, 

 aerial rooters, and those which support themselves by 

 prickles or hooks or by simply scrambling among other 

 and stiffer plants to reach the light. The Convolvulus 

 is a familiar example of the twiner, while the tendril- 

 bearing class may be represented by the Vine or the 

 Pea. The Ivy is a good representative of the aerial- 

 rooters, and in the Blackberry and the climbing Rose we 

 have representatives of the plants which support them- 

 selves by means of their prickles. Darwin looks upon 

 the tendril bearers as more highly developed than the 

 simple twiners, and it is interesting to observe in these 



