100 SOCIAL CONJUNCTIVE SYMBIOSIS 



grow hanging from telegraph wires if the air is sufficiently humid, 

 and many mosses and lichens grow equally well as epiphytes on 

 living hosts or upon rocks. When growing upon rocks they are called 

 lithophytes. 



Some epiphytes, however, are more or less limited to certain kinds 

 of hosts. The epiphytic fern, Polypodium polyyodioides (Fig. 46), 

 for example, is largely limited to trees having deeply-furrowed, soft 

 bark which has a high water-absorbing capacity and loses water 

 slowly. Such conditions are well provided by the American elm 

 ( Ulmus Americana) and the post oak {Quercus stellata). In the case 

 of epiphytic lichens it has long been known that the kinds that one 

 may expect to find on a tree depends upon the character of the bark; 

 whether it is rough or smooth, hard or soft, resinous or non-resinous, 

 etc. 



It is doubtful whether any distinct advantage of the epiphytic 

 habit can be noted. Often there is less competition for space than 

 there is among land plants but it is open to question whether this is a 

 real advantage, and in many cases, especially in the tropics, there 

 seems to be fully as much competition for space among epiphytes 

 as among other plants. A very obvious disadvantage is the unfavor- 

 able position with respect to water supply. 



Epiphytism frequently results in harm to the host plant. The 

 strangling action of the hemi-epiphytic fig has already been men- 

 tioned. Other epiphytes frequently break down their hosts by their 

 increasing weight or weaken them by cutting off a part of the light 

 or by interfering with the free interchange of gases. Epiphylls are 

 especially injurious in these latter ways. 



REFERENCES 



Harris, J. Authur: On the Osmotic Concentration of the Tissue Fluids of 



Phanerogamic Epiphytes, Am. Jour. Bot., 5, 490-506, 1918. 

 Hendricks, H. V.: Torsion Studies in Twining Plants II, Bot. Gaz., 75, 282- 



297, fig. 1-10, 1923. 

 Johnson, Duncan: Polypodium Vulgare as an Epiphyte, Bot. Gaz., 72, 



237-244, 3 fig., 1921. 

 Pessin, Louis J.: Epiphyllous Plants of Certain Regions in Jamaica, Bull. 



Torrey Bot. Club., 49, 1-14, pi. 1, fig. 1, 1922. 

 An Ecological Study of the Polypody Fern, Polypodium polypodioides, 



an Epiphyte in Mississippi, Ecology, 6, 17-38, 1925. 



