THE ROOT 
39 
hairs are replaced by a fungus whose hyphae absorb in the 
same way. This is termed a mycorhiza ; in the Fagaceae, 
Betulaceae, Coniferae, &c. it is ectoiropic , i.e. the hyphae do 
not enter the cells of the root but run between the epidermal 
cells, whilst in Orchidaceae, Ericaceae, &c. it is endotropic, 
the hyphae entering the cells. Mention may also be made 
of the tubercles of the roots of Leguminosae (q.v.) and other 
plants. In a few cases the roots are parasitic upon other 
roots living in the same soil, and are modified in structure 
to suit this modification of their absorptive functions (cf. 
Chap. III.). The second function, anchorage of the plant, is 
admirably effected by the branching and arrangement of the 
roots, aided by their internal mechanical adaptation, the 
vascular tissue being centrally placed so as best to resist 
a longitudinal strain. The growth in thickness of the root 
keeps pace with that of the stem. In many large trees the 
growth in thickness of the lateral roots at the base of the 
trunk is chiefly vertical, and ‘ buttress ’ roots are thus 
formed, e.g. in Ficus. 
A function often performed by roots, and one which is 
accompanied by serious modifications in their structure, is 
storage of reserve-materials to enable the plant to start 
growth in the wet season, or in the next spring, &c. (p. 33, 
and see Herbs in Chap. III.). The root is usually thickened 
to contain the materials. (See Brassica, Daucus, Raphanus, 
Taraxacum, &c.). 
In nearly all cases true roots perform normal root- 
functions and have normal structure, exhibiting little variety. 
In a few plants, e.g. Utricularia, Aldrovanda, Psilotum, 
Salvinia, there is no primary root, and its function (and 
often structure) is found in analogous organs (p. 32). In 
many other cases the primary root does not develope very 
far, and its functions are discharged by organs having the 
structure, physiology, and endogenous development of branch 
roots, but developed from the stem or leaves. These organs 
are not homologous by descent with true primary roots, and 
are classed as adventitious roots. In dealing with the 
homologies of these organs among themselves we can rarely 
go beyond the immediate circle of relationship of the plants 
in which they occur; the class is almost certainly poly- 
phyletic (p. 30) ; e.g. we have no right to say that the 
