16 A USTRA LI AN BOTANY. 



out a succession of fringe-like shoots from its stem, which 

 are termed aerial roots, from the fact of their growing in 

 the air — in this instance often in the crannies of walls and 

 buildings, acting as supports, especially where the structure 

 is decaying; and frequently covering the whole surface 

 with the plant. 



Parasitical plants — that is, plants living upon the nutri- 

 ment gathered by the roots of those upon which they are 

 fixed — have no true roots, their cell structure being so 

 infinitely blended with those of their host that these organs 

 are not necessary. The various species of Loranthus 

 (native mistletoe) which grace many kinds of trees, and 

 Cassytha (native scrub vine) rendering the Mallee scrub 

 occasionally almost impenetrable, are familiar examples. 



The roots of many water plants (aquatics) do not reach 

 the ground. In every case, however, the roots point down- 

 ward. 



Plate I. shows diagrams of the principal kinds of roots, as 

 follow : — 



Fig. i. Axial — a tap or true root of gum-tree (Eucalyptus). An 

 axial root, strictly denned, is a lengthening of the radicle into a tap 

 or main root. In some cases merely delicate fibres issue from it ; in 

 others — especially the aged tap-roots of large trees — the fibres attain 

 a great size. The axial or tap-root, however, maintains throughout 

 its character as the chief root. Dicotyledons (producing two cotyle- 

 dons or seed-lobes), except in a very few instances, have axial roots. 



Fig. 2. Adventitious root of wheat. When a root is not axial, it 

 is said to be adventitious. 1 Monocotyledons (producing one cotyledon 

 or seed - lobe), not possessing tap - roots, have adventitious roots. 

 The ivy, though its aerial roots are adventitious, belongs to the 



1 Adventitious — differing from the natural state of anything ; 

 organs produced in abnormal positions, as roots arising from aerial 

 stems. 



