38 



MORPHOLOGY OF THE ROOT. 



America (called Cursed Fig) are examples of the former. They 

 form trees, send down aerial roots in the manner of the Banyan ; 

 but, while some roots seek the ground, some may attach them- 

 selves to other trees parasitically, and draw from them a portion 

 of their support. The parasitism of certain herbaceous plants 

 with green foh'age is clandestine, the connection being under- 

 ground and therefore long unsuspected. This occurs in species 

 of Gerai'dia (at least of the section 

 Dasystoma) and other plants of the 

 same family, the uncultivability of 

 which is thereby explained. Also 

 in Comandra and in their relatives 

 the Thesiums of the Old World, 

 belonging to a natural order (the Santalaceae) which has much 

 affinity with the entirely parasitic order (Loranthacere) to which 

 the Mistleto belongs. 



64. Pale or Colored Parasites, such as Beech-Drops, Pine-Sap, 

 &c., are those which are destitute of green herbage, and are 

 usually of a white, tawny, or reddish 

 hue ; in fact, of an}" color except 

 green. These strike their roots, or 

 sucker-shaped discs, into the bark, 

 mostly that of the root, of other 

 plants, and thence draw their food 

 from the sap alread} 7 elaborated. 

 They have accordingly no occasion 

 for digestive organs of their own, 

 i. e. for green foliage. The Dodder 

 (Fig. 77) is a common plant of 

 this kind which is parasitic above 

 ground. Its seeds germinate in 

 the earth, but form no proper root : 

 when the slender twining stem 

 reaches the surrounding herbage, 

 it forms suckers, which attach 

 surface of the supporting plant, 

 penetrate its epidermis, and feed upon its juices ; while the 

 original root and base of the stem perish, and the plant has 

 no longer any connection with the soil. Thus stealing its nour- 



FIG. 76. Section of one of the attached rootlets of Gerardia, showing the union. 



FIG. 77. The common Dodder of the Northern States (Cuscuta Gronovii), of the 

 natural size, parasitic upon the stem of an herb : the uncoiled portion at the lower end 

 shows the mode of its attachment 78. The coiled embryo taken from the seed, con- 

 sisting of naked caulicle and plumule; moderately magnified. 79. The same in germi- 

 nation, elongating into a thread-like leafless stem. 



themselves 



