376 Relations between Species 



tissues of the tree, they are not classed as parasites. For the most 

 part the epiphytes do no harm to the host plant but occasionally they 

 become so numerous as to break it down or to stifle its growth (Fig. 

 10.8). Orange growers in Florida are forced to spend large sums 

 every year for the removal of Spanish moss from their trees. 



Fig. 10.8. Spanish moss {Tillandsia) growing in harmful abundance on a live 

 oak tree in South Carolina but practically absent from a neighboring pine. 



Some plants live as attached epiphytes on the surfaces of animals. 

 One extraordinary example is the green alga that grows on the long, 

 grooved hairs of the sloth. Since this alga often becomes sufficiently 

 abundant to give the animal a greenish appearance, the sloth pre- 

 sumably derives some advantage in concealment as it sleeps in the tree 

 tops. Equally remarkable in its habit is the green alga BasicUidia, a 

 genus of the Cladophoraceae, which grows only on the backs of fresh- 

 water turtles (Leake, 1939). The specific relation here appears to 

 be due to a dependence on keratin, since the alga can be cultured in 



