ORG.AJS'S OF NUTRITION. 



125 



obtain anr nourishment from it, biit must draw their food entirely 

 from the air in which they are developed, hence the name of air- 

 plants which is applied to them, Thev are also called epiphytes 

 because they commonly gi-ow itpon other plants. Most Orchids 

 {fig. 237) and Tillandsias afford us illustrations of epiphytical 

 plants. The roots of such plants are commonly green, and 

 possess a true epidermis and stomata ; in such particulars there- 

 fore, aerial roots also present exceptions to wliat is commonly 

 observed in other roots. The roots of most Orchids have also a 

 layer of usually very delicate cells filled with air over the true 

 epidermis, to which the name o{ root sheath (velamen radicuni) has 

 been applied by Schleiden, Avho also calls such roots coated roots. 



Besides these epiphytical plants there is another curious class 

 of plants which also grow upon others, which are called parasites. 



Parasites. — These are plants which not only grow upon others, 

 but which, instead of sending their roots into the air and 

 deriving their nourishment from it, as is the case with the 

 epiphytes, send them into the tissues of the plants iipon which 

 they are growing, and obtain their nourishment from them. 

 The Mistletoe ( Viscum album), Broom-rapes ( Orobanche), Dod- 

 ders {Cvscuta) {fig. 238), Bafflesia Arnoldi {fig 239), may be 



Fig. 238. 



cited as examples of such plants. These 

 parasites are of various natures ; thus 

 some have green foliage, as in the Mistle- 

 toe, &c. ; while others are pale, or possess 

 other tints than green, as the Broom- 

 rapes, Rafflesia, &c. The latter plant is 

 especially interesting as it produces the 

 largest flowers of any known plant : thus 



Fig. 239. 



Fig. 233. Cuscuta or Dodder Plant Fig. 239. Flower 



and bud of Bafflesia Arnoldi,- & parasitic plant of 

 Sumatra. 



the first flower that was discovered measured iline feet in cir- 

 cumference, and weighed fifteen pounds. 



Parasitical plants also vary in the degree of their parasitism; 

 thus the Mistletoe and the greater number are entirely dependent 

 upon the plants on which they grow for their nourishment; while 

 others, as the Dodder, obtain then- food at first bv means of the 



