MODIFICATIONS OF FORM 213 



The Climbing Habit. 



The biological advantage gained by the climbing habit is that the 

 plant which adopts it reaches the light with a minimum expenditure 

 upon its stem. A plant standing alone has to form a strong supporting 

 column. To do this requires a considerable expenditure of material 

 on tissues which are of little physiological use beyond giving mechanical 

 support. If then such support can be attained in some other way, 

 so much material will be gained. That the expenditure is really 

 saved by climbing plants is seen from their anatomy ; for their stems 

 show vessels relatively few and large, few other tissues of the wood, 

 and in herbaceous types, though cambium may be present, there is 

 an absence of tissue-masses formed by cambial thickening. There is, 

 however, a well-developed phloem, which in some cases is duplicated 

 on the side next the pith. The vascular strands thus constructed 

 contain little fibrous tissue, and are usually isolated one from another 

 by intervening tracts of soft parenchyma (compare Fig. 25, p. 48). 

 The result is that climbing stems are relatively weak and flexible, 

 while their leaves, flowers, and fruits may be large. These facts 

 demonstrate their dependence upon attachment to some stronger 

 support. 



The methods of climbing are various, and they are assumed by 

 representatives of many distinct families ; not uncommonly by isolated 

 species in a genus that does not climb as a rule. But in some families 

 of plants many genera and species are climbers, as in the Leguminosae, 

 Sapindaceae, and Bignoniaceae. The habit is much more frequent 

 in Dicotyledons than in Monocotyledons. Several Ferns have also 

 adopted a very successful climbing habit. This widespread and often 

 isolated occurrence of climbing, as well as the variety of the methods 

 involved, suggests that the habit has been acquired along many 

 distinct lines of Descent. Instances of marked homoplasy are numerous. 

 While climbing is common in our native Flora, it is most frequent here 

 in herbaceous plants, such as Vetches, Convolvulus, or Hop. They may 

 be annuals, like the Black Bindweed ; or perennials with an under- 

 ground root-stock, like the Hop, or Black and White Bryony. Some 

 few are woody, as the Honeysuckle, and Clematis and Ivy. While this 

 is less common in temperate Floras, it becomes a very marked feature 

 of Tropical Forests. There the huge woody " lianes " develop their 

 leafy shoots high up amid the branches of the lofty canopy of trees, 

 while their flexible but woody stems hang down like ropes, connecting 

 the shoot above with the root-system in the soil. But such climbers 



