BOOK II. 
SPPXIAL MORPHOLOGY 
AND 
OUTLINES OF CLASSIFICATION. 
GROUP I. 
THALLOPHYTES. 
In this group are comprised Algae, Fungi, and Lichens, the term bemg appUed 
to them because their vegetative body is usually a Thallus, i.e, exhibits no differ- 
entiation into stem, leaf, and root, or, if at all, only in a very rudimentary 
degree. There occur however in various groups of Thallophytes transitions from 
the simplest forms, which display no external differentiation, to others which show 
some indication of it ; in the most highly developed representatives of some 
groups the external differentiation is carried so far that the terms leaf and stem 
are as applicable to them as to the higher plants, A true root, in the sense in 
which the term is applied to vascular plants, is however never found, though root- 
like organs are commonly present which are termed Rhizoids ; these are however 
always distinguishable by the absence of a root-cap and by the branching not being 
endogenous. 
Like the external, the internal differentiation of Thallophytes also begins at 
the lowest stages, ascending by numberless transitional steps to a more perfect 
development of cells and tissues ; but even in the most perfectly developed forms we 
do not meet with any sharp differentiation into those different systems which we 
know among the higher plants as epidermal tissue, fundamental tissue, and fibro- 
vascular bundles. Even where the thallus consists of large masses of tissue, as in 
Fungi, it is still strikingly homogeneous. 
Thallophytes nevertheless present a great variety of examples of the mode in 
which morphological differentiation proceeds from the simplest organic forms to 
