STEMS 



659 



( ^l water. The absorptive organs of epiphytes, as seen in the lichen 



I thallus, in the aerial roots of orchids, and in the leaves of mosses and 



I bromelias, have been treated elsewhere. Most orchids and bromelias 



I have highly cutinized epidermal walls, which, with other protective 



I features, reduce transpiration to such an extent that the plants do not 



I dry out for weeks. In many orchids the leaves or stems or both are 



I greatly thickened and contain large quantities of water, the stems fre- 



l quently showing bulbous'enlargement. In most bromelias the leaves 



| form a sort of cistern, which retains water for 



weeks after a rain; since these cistern epiphytes 



absorb water almost entirely through the leaves, 



the advantage of the habit is obvious. 



Epiphytts ; injury due to epiphytes. Epiphytes 

 occurring on leaves are known as epiphylls (fig. 970), 

 and are especially characteristic of the moist tropics; 

 lichens, mosses, and even vascular plants occur in this 

 strange position. Epiphylls are very injurious to the 

 leaves on which they grow, cutting off light and im- 

 peding gas exchange. Epiphyllous lichens show all 

 gradations between epiphytism and parasitism, some 

 forms being strictly external to the leaf, while other 

 forms destroy the cuticle and thus have a position 

 directly on the outer wall of thepidermis ; in still other 

 cases (as in Strigula) the lichen tissues may penetrate 

 the mesophyll. Stem epiphytes also often injure their 

 supporting plants, checking gas exchanges through the 

 bark or breaking the branches by their weight. Even 

 in cold climates the beard lichens (such as U nea and 

 Alectoria) may enfold the leafy twigs of conifers to such 

 an extent as to cause their death ; in some cases the hyphae of Usnea penetrate the 

 living cells of the supporting plant. 



Subordinate categories of epiphytes. Plants epiphytic for only a part of their 

 existence (as Ficus, p. 515) are known as hemi-epiphytes. In temperate regions many 

 ordinary soil plants are found in the crotches of trees, where a little soil has collected; 

 such plants may be called pseudoepiphytes. Various algae occur as epiphytes in 

 the water, but most forms grow equally well when attached to rocks or shells; 

 however, for species requiring considerable light, attachment to plant organs which 

 float near the surface may be advantageous. Various epiphytic lichens and mosses 

 occur also as lithophytes, and Tillandsia may grow even on telegraph wires. 



Restriction of epiphytes to particular supports. Most true epiphytes are re- 

 stricted to trees, some being confined to particular species. Among the lichens 

 some crustose species (as Graphis scripta) grow chiefly on smooth-barked trees. 

 The palmetto has certain characteristic epiphytes that rarely if ever grow on other 

 trees; probably this is because its soft spongy bark especially facilitates attachment. 



Fie. 970. Epiphyllous 

 foliose lichens (Strigula 

 complanata) on a leaf of 

 Ocotea; a, fruit dots (apo- 

 thecia) ; such lichens prop- 

 agate radially and the 

 largest one here figured 

 already has disorganized at 

 the older central portion, 

 the thallus becoming ring- 

 shaped instead of solid. 



