DIV. I 



MORPHOLOGY 9 



in the course of descent its METAMORPHOSES. One of the most 

 important results of morphology is the demonstration THAT THE 



VARIOUSLY FORMED PARTS OF EVEN THE MOST HIGHLY DIFFERENTIATED 

 PLANTS ARE TO BE TRACED BACK TO A FEW PRIMITIVE FORMS. Those 



organs which have developed phylogenetically from a common primitive 

 form are spoken of as HOMOLOGOUS, however different they may appear. 

 The same morphological value is ascribed to them. For example, 

 foliage leaves and the leaves of the flower (sepals, petals, stamens, and 

 carpels) are homologous, and this extends to the leaf-tendrils (Fig. 209) 

 and the leaf-thorns (Fig. 197). Organs of completely different 

 structure and functions can thus be homologous. On the other hand, 

 organs with similar construction and functions (e.g. tubers (Figs. 203, 

 205, 206), thorps (Figs. 197-199), tendrils (Figs. 208-210)) have 

 often been genetically derived from different primitive forms. Such 

 organs are spoken of as ANALOGOUS (for examples cf. p. 165 ff). 



Little differentiated structures with ill-defined functions, which we 

 have reason to believe will in the future become transformed into 

 more complete organs with well-marked functions, are termed RUDI- 

 MENTARY organs. Incomplete structures which have retrograded 

 from more perfect ones are REDUCED organs. 



3. Lastly, it is an aim of morphology to ascertain the causes or 

 conditions which underlie the processes of external and internal 

 differentiation of the plant and its parts, and of their inherited 

 (phylogenetic) transformations. In this way it may be possible to 

 ascertain clearly how in the course of descent adaptive, characters 

 have arisen. The study which concerns itself with such questions is 

 EXPERIMENTAL MORPHOLOGY. Most of the problems of this are more 

 conveniently dealt with as a section of physiology in relation to the 

 other vital processes of the plant (developmental physiology or 

 mechanics of development). 



Morphology may be divided into external morphology and internal 

 morphology or anatomy. Such a division would not, however, be 

 suitable here, when it is desirable to regard the parts of the plant 

 as organs with definite functions. For this it is necessary to show 

 the intimate connection that frequently exists between the function 

 of an organ and both its form and internal structure. From the 

 outset we must be concerned with the plant as a living organism and 

 not as a dead structure. The first question to be faced is with what 

 life is most intimately connected, and this proves to be with a part 

 only of the whole substance of the plant, namely, with the protoplasm. 

 The protoplasm is, as a rule, enclosed in the cells which can be regarded 

 as the elementary parts of the organism. The part of morphology 

 which is concerned with the structure of cells is termed CYTOLOGY 

 and will be dealt with first. The tissues formed by associated cells 

 will then form the subject of a second part of morphology to which 

 the name HISTOLOGY is given. Lastly, ORGANOGRAPHY deals with 



