116 ELEMENTS OF PLANT ANATOMY. 
6. Comparative Anatomy of the Root. 
We come now to the consideration of an organ about whose 
general structure and function there is no possible ambiguity. 
Regarding its morphological value, however, there is room for 
a difference of opinion. It has been customary to speak of all 
organs whose function is to hold the ordinary plant to the soil 
or the parasitic plant to its host and which conduct food from 
these to the plant, as either true roots or root-like organs. To 
the latter class belong all such organs of plants below the vas- 
cular cryptogams. True roots occur then first in the vascular 
cryptogams and are common to all plants from this group up- 
ward. There are, however, a few even of the highest class of 
phanerogams which have no root. The most prominent distinc- 
tion made by botanists between the root-like organ (rhizoid) and 
the true root is that the true root is always tipped with a root- 
cap. The presence or absence of this cap determines the rank 
of the organ, though functionally both classes are the same. If 
we Inquire after the reason of this apparently arbitrary test, it 
will be seen that there is some ground for this distinction with 
which the root-cap itself has no connection, except that it is 
always on roots having a certain origin. It is therefore only a 
sign of their method of origin. 
If we go back to the simpler forms of plants to find the first 
traces of root-like organs, we may perhaps begin with the haus- 
toria which grow on the mycelium of certain fungi. A little 
higher up in the scale, we reach what may be taken as the 
normal type of this organ in all plants below vascular crypto- 
gams, namely: the rhizoids of the lichens, Hepaticae, and 
Musci. 
The transition from thallus to cormophyte is said to begin 
in the group Hepaticae and to reach its full completion in the 
next higher group, the Musci. If we compare a thalloid plant 
of the former class with a cormophyte of the latter, we find that 
