GENERAL MORPHOLOGY OF THE PLANT 25 



because they commonly grow upon other plants. They have 

 frequently also small roots of the ordinary type, which penetrate 

 into cracks or crannies in the bark of the supporting plant, from 

 which they absorb small quantities of food from the debris which 

 accumulates there. Many of the tropical Orchids [fig. 33) and 

 Tillandsias afford us illustrations of epiphytic plants. The 

 aerial roots of such plants are frequently green and serve as 

 organs of assimilation. The aerial roots of Orchids have also a 

 layer of usually very delicate fibrous cells covering them, to 

 which the name of velamen has been applied. 



Boots of Parasites. — These are plants which not only grow 

 upon others, but which, instead of sending roots into the air and 

 deriving their food from it, as is the case with the epiphytes. 

 Fig. 34. 



Fig. 3.5. 



Fig. 34. Cuscuta or Dodder-plaut. 



Fig. 35. Flower and flower-bud of 

 Rafflesia Arnoldi, a parasitic plant of 

 Sumatra. 



send peculiar root-like bodies into the tissues of the plants upon 

 which they grow, and obtain nutriment from them. The plant 

 which they thus penetrate and feed upon is termed their host ; 

 and the sucking roots are called haustoria. They differ from 

 true roots in the manner of their origination, as we shall see 

 later. The Mistletoe (ViscuTn album), Broom-rapes {Orobanche), 

 Dodders (Cuscuta) {fig. 34), and Rafflesia Arnoldi {fig. 85), 

 may be cited as exainples of such plants. These parasites 

 differ from each other in appearance ; some have green foliage, 

 as the Mistletoe ; while many others are pale or brownish or 

 possess other tints than green, as the Broom-rapes and Rafflesia. 

 Parasitic plants also vary in the degree of their parasitism ; 

 thus the Mistletoe and the greater number of parasites are, so 



