184 



BOTANY 



PART I 



-st 



(e.g. in the Orchidaceae), being replenished in moist periods, or there 

 may be adaptations to catch water more directly. 



In our latitudes epiphytes are represented only by some Algae, Lichens, and 

 Bryophyta growing on the bark of trees. In the tropics, however, owing to the 

 humidity of the atmosphere and the frequent and heavy downpours of rain, many 

 cormophytes live as epiphytes. These plants, which belong especially to the 

 Pteridophyta and the families Orchidaceae, Bromeliaceae, and Araceae, have no 



connection with the 

 water-supply in the soil. 

 Their difficulty in 

 obtaining water explains 

 why the tropical epi- 

 phytes are nearly all 

 well-marked xerophytes 

 (Fig. 201). They are 

 fastened by ATTACHING 

 ROOTS which are rela- 

 tively short, unbranched, 

 and negatively helio- 

 tropic, and grow round 

 and clasp the branch on 

 which the plant grows. 

 In addition to these 

 attaching roots, much 

 longer ABSORBENT ROOTS 

 are found in many 

 Araceae, hanging down 

 freely in the air without 

 branching until they 

 reach the soil. Most 

 epiphytes, however, are 

 dependent on the rain- 

 fall for their water- 

 supply, and frequently 

 have special arrange- 

 ments for collecting and 



FIG. 213. A, Dischidia liajflesiana with foliage leaves (Z)and pitcher 

 leaves (fc). B, Pitcher cut longitudinally ; o, opening ; st, stalk ; 

 w, root. (A about J, B about \ nat. size. After TREUB.) 



retaining this. The 

 many-layered epidermis 

 of the aerial roots of 

 many Orchids, and of 

 various Aroids, under- 

 goes a peculiar modifica- 

 tion and forms the so-called VELAMEN, a parchment -like sheath surrounding 

 the roots, and often attaining a considerable thickness. The cells of this 

 enveloping sheath are generally provided with spiral or reticulate thickenings, 

 and lose their living contents. They then become filled with either water or 

 air, depending upon the amount of moisture contained in the surrounding 

 atmosphere. These root -envelopes absorb water like blotting-paper; when the 

 velamen is filled with water the underlying tissues impart a greenish tint to the 

 root ; but if it contains only air the root appears white. In other epiphytic 

 Orchidaceae and Araceae there are upwardly-directed roots forming a branched 



