MORPHOLOGY 15 



the similarity of their organisation is due to true homology. In 

 addition to the distinction of stem and leaf in their shoots, they possess 

 true roots, while even the most highly organised Bryophyta have 

 only filamentous structures (rhizoids) (Fig. 11 r) in place of roots 

 to attach them to the substratum. True roots, on the other hand, 

 which appear for the first time in the Pteridophyta, are, for the most 

 part, cylindrical structures with apical growth. Besides possessing 

 a distinctive internal construction they are distinguished in their 

 external form from the shoot by having a special sheath, the ROOT- 

 CAP or CALYPTRA covering the growing point, and by the absence 

 of leaves. The body of the higher plants provided with stem, 

 root, and leaf is termed a cormus, and the plants are spoken of as 

 cormophytes. 



The Metamorphosis of the Primary Members of Cormophytes. 

 After the differentiation into stem and leaf and the appearance of 

 the roots had taken place, further changes have consisted essentially 

 in a more or less profound modification of these primary members 

 of the cormophytic plant-body. Such changes are spoken of as a 

 metamorphosis (p. 10), and in some cases may be so extensive as to 

 lead to one primary member assuming the characters of another. 



The relationships between homologous members, which are often vei'y striking, 

 did not escape the notice of earlier observers. They suggested comparisons, 

 although no real phylogenetie basis for such comparisons existed. Thus, an ideal- 

 istic conception of the form of external members was developed, and finally reached 

 its highest artificial development in GOETHE'S Theory of Metamorphosis ; and its 

 scientific conclusion in the writings of ALEXANDER BIIAUN. As the great variety 

 exhibited in the external appearance of the lower plants precluded any possibility 

 of assigning to them hypothetical primitive forms, the whole terminology of the 

 external morphology of plants has been derived from conceptions applicable only 

 to the Cormophytes. Even to-day, the same terms used in reference to the 

 Cormophytes are applied to parts of the Thallophytes, which are evidently only 

 analogous. 



Members of Independent Origin. Parts which cannot be derived 

 by metamorphosis of the primary members of the cormophytic plant 

 are sometimes met with. Though they are of infrequent occurrence 

 they are of importance as showing that the natural evolutionary 

 process is not to be limited by any formal scheme.. Such structures 

 will be discussed farther on. 



Relations of Symmetry 



Every section through a part of a plant, made in the direction of 

 its longitudinal axis, is distinguished as a longitudinal section ; those 

 at right angles to it being termed cross or transverse sections. 

 Parts of plants which may be divided by a number of longitudinal 

 planes into like halves are termed either MULTILATERAL, RADIAL, or 

 ACTINOMORPHIC. Such parts are symmetrically constructed around 



