CHAPTER VIII 

 SOCIAL CONJUNCTIVE SYISIBIOSIS 



Conjunctive symbiosis includes all cases of the living together 

 of two or more unlike organisms in which the symbionts are in 

 actual contact throughout the time during which they are said to be 

 in a state of symbiosis. Any example of conjunctive symbiosis in 

 which there is no direct food relation between any of the symbionts 

 may be classed as social. The commonest examples of social con- 

 junctive symbiosis are those of the various kinds of climbing plants 

 and of plants that grow upon other plants for support but without 

 deriving food from their hosts. 



55. Lianas.— Lianas are plants that climb by one means or 

 another (Fig. 36) . They do not have a sufficient amount of mechani- 

 cal tissue to maintain themselves in an upright position and so are 

 forced to climb upon some support in order to display their foliage 

 in an adequate way. If the support happens to be another plant, 

 as it does in a great majority of cases, the phenomenon represents a 

 svmbiotic relation between the liana and its host. 



The number of species of lianas in the north temperate zone is 

 comparatively small but in tropical forests, where they reach their 

 maximum development, the wealth of species is astonishingly great. 

 Certain families, such as the legume, moonseed, soapberry, and 

 Bignonia families are especially rich in climbing species, but a very 

 large number of other families contain some lianas. In some tropical 

 forests the lianas rank next to the trees themselves in conspicuousness. 

 They vary greatly in size and in habit. They may be closely twisted 

 around large or small stems or they may hang as streamers from the 

 branches of trees. Some are stretched tightly from one tree to 

 another while others loop downward to form great festoons from one 

 branch to another. In many cases, too, they occur tangled together 

 in masses on the ground. 



The internal structure of the stems of lianas is characteristically 

 different from that of upright stems. Especially is this true of 

 twining plants the stems of which are necessarily pliable. In such 

 stems the wood is seldom continuous but is separated into distinct 

 wedges which frequently become much lobed and cleft as secondary 

 growth proceeds. Sometimes the M^ood is also separated radially, 



(89) 



