THE LIANAS 



We must look for the reason in the more brilliant illumination of 

 the tropical forest, the greater number of points of support offered 

 to the climbers, and the much greater humidity of the atmosphere. 

 In our pine- or beech-woods the climbing plants — as Haberlandt 

 has remarked — do not obtain more light by climbing higher, and 

 if they grow as high as the top of a tree they pass, first of all, into 

 complete shadow. In the better lighted tropical woodland, which is 

 more uniform in structure at all levels, the lianas can find sunlight 

 at any height. And this more uniform structure provides the liana 

 with rungs by which it can climb, step by step, higher and higher. 

 I have already remarked that the branches of tropical trees bend 

 upward more sharply than those of our Northern trees, so that they 

 offer all sorts of angles as points of support. Moreover, the sparser 

 foliage of the tropics is more easily penetrated. 



Again, the importance of more frequent rainfall in the development 

 of the lianas must not be underestimated ; it has, as a matter of fact, 

 long been recognized. Observation shows that the woods of the drier 

 tropical countries are poorer in lianas. 



As a matter of fact, the problem of conveying the moisture which 

 is taken from the soil through the long, thin stems is not an easy 

 one to solve. Rattan lianas have actually been measured which were 

 780 feet in length, with an almost uniform thickness of an inch and 

 a quarter to an inch and two-thirds ! And when these plants unfold 

 their leaves in the brilliant sunlight far overhead, the moisture 

 evaporates all the quicker. For plants, of course, transpire, as every- 

 one knows who has kept them indoors in winter in order to restore 

 the humidity of air dried by artificial heating. The moisture lost 

 by the leaves lias to be replaced, and quickly, or the leaves and 

 shoots would wither. 



In order to convey the stream of water from the roots to the stem, 

 branches and leaves, the body of the plant develops vascular con- 

 duits. These, of course, are of microscopic diameter, and if we tear 

 the body of the plant asunder we can distinguish them only as 

 fibrous strings, as they are provided with supporting fibres, and 

 combine with them to form fibro-vascular bundles; in the leaves 

 these appear as ribs, and divide to form the familiar vascular 

 network. 



If we now cut through the stem of a liana — one of the Brazilian 

 "orchid" lianas — we can perceive with the naked eye the sieve-like 

 perforations of the resected surface visible as tiny dots (Fig. 3) , for in 

 the lianas the vascular channels are unusually large, in order that 



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