SUBMERGED PLANTS : EPIPHYTES 71 



attached to other plants, most often dinging to, or rooting in, 

 the bark of trees. In the tropical rain forests epiphj^tes reach 

 their highest degree of luxuriance, the stems, branches, 

 and even leaves of the trees being covered by a host of 

 species and individuals. Outside the tropical rain forest 

 true epiphytic communities are found in the temperate forests 

 of Chili and New Zealand. The less highly specialised 

 epiphytes may grow in crevices of the bark, in clefts of the 

 trunk, in the crooks of branches, where accumulations of 

 dust, humus, and other debris furnish them with a limited 

 amount of soil. But the most characteristic cling to their 

 hosts by tendrils or by roots, these frequently forming an 

 extensive network round trunk and branches. Sometimes, 

 as in some epiphytic species of Ficus, special roots descend 

 to and penetrate the soil ; many roots are dependent on such 

 supplies of water as they can get on the trees ; sometimes 

 they hang freely in bunches or singly in the air, wholly 

 dependent on atmospheric precipitation. These subaerial 

 roots have in general lost their geotropism. They are often 

 strongly haptotropic, negatively phototropic, and hydro- 

 tropic. They are usually unbranched, though if they enter 

 the soil they may branch there. The growing zone is often 

 of considerable length. 



Roots with Velamen. — In many epiphytic orchids and 

 aroids the roots have a special absorbing tissue lying outside 

 the cortex and termed the velamen. This is morphologically 

 equivalent to the epidermis, but it is usually several cells 

 thick. The cells are dead, thin walled, and empty, and 

 are prevented from collapsing by fine spiral or netted 

 thickenings. They are open to each other and to the outside 

 by pores. Within this velamen lies the living cortex, the 

 cells of which frequently possess chloroplasts ; its external 

 layer is specialised as an exoderniis, most of the cells having 

 thickened and suberised walls impermeable to water. 

 Here and there a transfusion cell of the exodermis remains 

 thin walled and serves to pass water inwards. When dry, 

 the velamen has a silvery, parchment-like appearance. 

 On wetting, it absorbs water greedily and rapidly, the pores 



