754 RELATION OF STRUCTURE TO FUNCTION IN ROOTS. 



between subterranean and aerial roots vanishes, and, as in other similar cases, it 

 becomes evident that all these classifications are but artificial. 



Parasitic roots (radices parasiticce) grow down into the living tissue of the 

 host-plant and absorb from it the materials needed by them, and by the plant to 

 which they belong, for further development. They are sometimes called haustoria. 

 They are either wart-like, disc-shaped, or spherical in outline, or assume the form 

 of sinkers; occasionally they remind one of a hyphal net- work. Sometimes they 

 spring laterally from an epigeal, sometimes from an underground stem. They also 

 frequently proceed as lateral members from underground roots. Their structure 

 and various developments were so fully described on pp. 173-213 that we need 

 only now refer to what was there stated. 



Roots, whose especial province it is to maintain a plant in the position it has 

 once assumed, may be distinguished as clinging and as supporting roots. Clinging 

 roots (radices adligantes) really comprise all roots whose absorbent cells are so 

 closely united with the substratum that a displacement can only be brought about 

 by the exertion of considerable force. Even floating roots, inasmuch as they adhere 

 to the water and so give a certain amount of stability to the whole plant, may be 

 regarded as clinging roots. The duckweeds (Lemna minor, polyrrhiza, gibba), 

 whose long, spirally - twisted, fascicled roots grow down into the water, are not 

 easily moved by wind from the position they have taken up. Plants are of course 

 still better fixed in the substratum by subterranean roots which adhere to the 

 solid particles of the soil by means of their root-hairs. By this union of roots 

 and earth -particles we get a compact mass, difficult to break up, and it is well 

 enough known that loose soil may be solidified by plants possessing much-branched, 

 wide-spreading roots, and that certain grasses are made use of to bind shifting 

 sands together. When clinging roots are mentioned in plant descriptions, those 

 in particular are referred to which firmly connect epigeal portions of stems to any 

 support, as, for example, the short, climbing roots of the Ivy, or of Tecoma 

 radicans, the much-branched roots which cover stones and the bark of trees with 

 their net-works, the adherent roots of numerous species of Bignonia and Cereus, 

 and the ribbon-like roots of certain tropical orchids which have fastened to the 

 bark of trees, especially those of Phalcenopsis Schilleriana, described on p. 108; 

 and finally the girdle-like roots of Ficus and Wightia, figured on p. 705. 



Supporting roots, as their name implies, have the task of supporting the stems 

 to which they belong. They are always visible above-ground, and assume the form 

 of buttresses when they spring from erect trunks, of pillars when they belong to 

 the projecting lateral branches of a stem. They may be conveniently divided into 

 tabular, stilt-like, and columnar roots. Tabular roots (radices parietiformes) proceed 

 from the lower part of an erect trunk, and have the form of flanges placed on end. 

 They may also be compared to massive planks of wood used for fencing in roads. 

 They radiate out in all directions and give to the approaches to the main trunk the 

 appearance of short precipitous valleys which become gradually narrowed and 

 terminate blindly in an acute angle. The tabular roots frequently resemble narrow 



