ORGANS OF NUTRITION. 



119 



Orchids have also a layer of usually very delicate cells filled 

 with air over the true epidermis, to which the name of root sheath 

 has been applied by Schleiden, who 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 deriv- 

 ing their nourishment from it, as is the case with the epiph}i;es, 

 send them into the tissues of the plants upon which they grow, 

 and obtain their nourishment from them. The Mistletoe ( Vis- 

 cum album), Broom-rapes {Orobanche), Dodders (Ci(scuta), {fig. 

 231), Eafflesia Arnoldi {fig. 232), may be 

 J^tg. z6l. cited as examples of such plants. These 



parasites are of various natures ; thus 

 some have green foliage, as in the Mistle- 

 toe, while others are pale, or possess 

 other tints than green, as the Broom- 

 rapes and Eafflesia. The latter plant is 

 especially interesting as it produces the 



Fig. 232. 



Fig. 231. Cusruta or Dodder Plant. Fig. 232. Flower 



and flower-bud of Eafflesia A-nioldi, a parasitic 

 plant of Sumatra. 



largest flowers of any known plant : thus the first flower that 

 was discovered measured nine feet in circumference, and weighed 

 fifteen pounds. 



Parasitical plants also vary in the degree of their parasitism ; 

 thus the Mistletoe and the greater number of parasites are 

 entirely dependent upon those on which they grow for their 

 nourishment ; while others, as the Dodder, obtain their food at 

 first by means of the ordinary roots contained in the soil, but 

 after having arrived at a certain age these perish, and they then 

 derive it entirely from the plants upon which they grow ; others 

 again continue throughout their life to derive a portion of their 

 nourishment by means of roots imbedded in the soil. 



We have now described the general characters and structure 

 of the trice or primary root, and the adventitious or secondary root. 



