SECT. I MORPHOLOGY 15 



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 Metamorphosis of the Primary Members of Cormophytes. — 

 After the differentiation into shoot and root had taken place, further 

 changes have consisted essentially in a more or less profound modifica- 

 tion of these primary members of the cormophytic plant-body. Such 

 changes are spoken of as a metamorphosis (p. 10). 



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. 



The relationships between homologous members, which are often very striking, 

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

 although no real phylogenetic liasis for such comparisons existed. Thus, an 

 idealistic conception of the form of external members was develo23ed, and reached 

 its scientific conclusion in the writings of Alexander Beaun. 



As the great variety exhibited in the external appearance of the 

 lower plants precluded any possibility of assigning to them hypo- 

 thetical primitive forms, the whole terminology of the external 

 morphology of plants has been derived from concej^tions 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. On this account in treating here 

 of the nature and aim of general morphology examples will be taken 

 mainly fi'bm the Cormophytes. 



Relations of Symmetry 



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

 from base to apex, 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 actino- 

 MORPHIC. Such parts are symmetrically constructed around their 

 longitudinal axis. The degree of symmetry peculiar to any leafy 

 shoot will be more apparent from a diagram, that is if the leaves 

 which it bears be projected on a plane at right angles to its axis. 

 The radial symmetry of a shoot with opposite leaves in alternating 

 whorls is clearly shown in the adjoining diagram (Fig. 13a). A 



