BOOK II. 



SPECIAL MORPHOLOGY 



AND 



OUTLINES OF CLASSIFICATION. 



GROUP I. 

 THALLOPHYTES. 



In this group are comprised Algae, Fungi, and Lichens, the term being applied 

 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 tliose 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 



