CHAP, xxv SYMBIOSIS OF PLANTS WITH ONE ANOTHER 91 



to hold firmly to their supports, but also by suitable internal structure 

 to solve the problem of conduction of material, as well as other problems 

 that they owe to the length and slenderness of their stems. 1 Lianes 

 display very different degrees of adaptation to the climbing habit. The 

 lowest stage is represented by : 



1. ' Semi-lianes ' (Warming) and scramblers (Schenck), which occur 

 particularly at the margins of forests, in hedges, and in bushlands. 



More elaborate and specialized in adaptation are : 



2. Root-climbers, which can clamber up thick tree-trunks and rocks. 



3. Twining plants. 



4. Lianes equipped with tendrils or other irritable organs, and capable 

 of embracing slender parts of plants. These were termed by Darwin 

 ' hook-climbers ', * tendril-bearers,' and ' leaf-climbers '. 



It is characteristic of the majority of lianes for the leaf to be broad, 

 more or less cordate, and long-stalked. 2 (But to this rule exceptions 

 are provided by those Papilionaceae and other lianes that climb by 

 means of tendrils on the ends of their leaves.) In structure of leaf and 

 stem some lianes recall xerophytes ; it seems quite natural that lianes 

 should be exposed to the possibility of losing more water by transpiration 

 than can be balanced by supplies from the root, and hence should be 

 structurally adapted to provide for this contingency. 3 Certain species, 

 for instance, of the genus Ficus occur both as lianes and as epiphytes. 4 

 The liane-form is a consequence of the congregation of plants to form 

 communities, but lianes are partially independent of their fellows, since 

 inanimate supports will sometimes serve them quite as well as living ones. 

 Lianes belong especially to certain families, including Ampelidaceae, 

 Asclepiadaceae, Apocynaceae, Bignoniaceae, Cucurbitaceae, Papiliona- 

 ceae, Sapindaceae, Dioscoraceae, and others. 



CHAPTER XXVI. COMMENSALISM. PLANT-COMMUNITIES 



IN the preceding chapter we have dealt with the different bonds that 

 may link plants together, and one individual with another : parasite 

 with host, master with slave (helotism of lichens) ; we then discussed 

 mutualists, epiphytes, and finally species associated with the whole 

 plant-community. We have yet to consider the great, highly complex 

 plant-communities which form the essential foundations of oecological 

 phytogeography. 



The term ' community ' implies a diversity but at the same time 

 a certain organized uniformity in the units. The units are the many 

 individual plants that occur in every community, whether this be a beech- 

 forest, a meadow, or a heath. Uniformity is established when certain 

 atmospheric, terrestrial, and any of the other factors discussed in Section I 

 are co-operating, and appears either because a certain, defined economy 

 makes its impress on the community as a whole, or because a number of 



1 For further details see C. Darwin, 1875 ; Schenck, 1892, 1893 ; Warming, 1892 ; 

 A. F. W. Schimper, 1898. * Lindman, 1899; Warming, 1901. 



3 Warming, 1892. * Schenck, 1892, 1893 5 Whitford, 1906. 



