LEAVES AND LEAF-FORMING AXES. 



131 



their earliest states the members to which the same morphological names are applied 

 [e. g. all the leaves of a plant) are extremely similar to one another ; at a subsequent 

 period all those distinctions arise which correspond to their different functions. 

 With reference to these relationships we may now obtain a definition of Metamor- 

 phosis which can be used in a scientific manner: — Metamorphosis is the varied 

 development of members of the same morphological significance resulting from their 

 adaptation to definite functions. 



(a) The conceptions of Stem, Leaf, Root, Trichome, as at present employed in botany, 

 result from the consideration of highly developed plants the different members of which 

 actually present considerable diversities, from a purely formal point of view; but if the 

 attempt is made to apply these conceptions in the same manner to the less differentiated 

 plants, HcpatiCcT, Algir, Lichens, and Fungi, many difficulties arise, depending principally 

 on the fact that the members of the thallome sometimes display striking resemblances to 

 leaves, hairs, stems, and even roots, while other characteristics of these parts are absent. 

 In a word, transitions occur from the members of Thallophytes which are morpho- 

 logically but slightly diiferentiated to the highly differentiated members of Cormophytes. 

 In the members which we term stem, leaf, root, hair, it is clear that those differences are 



.only augmented which also occur, though in a lesser degree, in the more homogeneous 

 ramifications of the thallome especially of the higher Algee ; absolute distinctions be- 

 tween thallomes and leaf-bearing axes are not %o be found. It is therefore a matter of 

 convenience where the boundary-line is drawn. 



(b) The expressions Thallome, Caulome, Phyllome, IVichome, Root, designate, as has 

 been said, several ideas, from the definition of which are eliminated all those properties 

 of the members which are calculated only for definite functions, while a few character- 

 istics only, which concern their origin and mutual position, are kept in view. Parts 

 which are physiologically entirely different may therefore be morphologically equi-valent, 

 and, 'vice versa, physiologically equivalent organs may fall morphologically under quite 

 different conceptions. The statement, e. g. that the sporangia of Ferns are trichomes, 

 means only that their origin, like that of all hairs, is from the epidermis-cells ; in this 

 characteristic hairs and the sporangia of Ferns are morphologically equivalent. On the 

 other hand the underground hairs of Mosses and true roots are physiologically equi- 

 valent ; both serve for the absorption of nourishment and the fixing of the plant in the 

 ground, although the former fall under the morphological conception of trichomes, the 

 latter of roots. 



(c) General ideas, like those considered here and in the sequel, depend always on 

 abstractions, the practical clearness of the particular ideas from which they have been ob- 

 tained by abstraction is therefore necessarily wanting in them. How far the abstraction 

 may be carried is more or less arbitrary ; and the only correction for this arbitrariness 

 lies in a reference to the usefulness of the idea for scientific reasoning. Those ideas are 

 the most useful which, from the greater precision of the definition, and from their 

 greater clearness, include the greatest possible number of particular cases ; for in this 

 manner is that complete general comprehension of the phenomena most easily obtained 

 which must precede a closer examination of them. The definitions in the following 

 paragraphs are given from this point of view. 



Sect. 21. Leaves and Leaf-forming Axes\ — The members of the plant 

 which are called Leaves (Phyllomes) in Characeae, Mosses, Vascular Cryptogams, and 



^ Niigeli u. Schwendener, Das Mikroskop, pp. 599 et seq. Leipzig 1869.— Hofmeister, Allge- 

 meine Morphologie der Gcwebe, sect. 2. Leipzig 1S68. — ^Pringsheim, Jahrb. fiir wissen. Bot. III. 

 p. 484. Derselbe iiber Utricularia. Monatsber. der Berliner Akad. Feb. 1869. — Hanstein, Bot. 

 Abhandlungen. Heft. I. Bonn 1870.— Leitgeb, Botan, Zeitg. no. 3, i"i7i. 



K 2 



