EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 347 



and each is capable of indefinite amplification and of the production 

 of parts similar to itself. Since these regions of the plant are distinct 

 in origin, and in their relation to the whole plant-body, no homology 

 can be recognised between them or tfreir parts. The distinction 

 between these regions seen in the Phanerogamic seedling is of wide 

 application. Root and Shoot constitute the fundamental categories of 

 parts seen in Vascular Plants, and to one or other of them all the parts 

 of the vegetative system may be referred. 



The Root presents few morphological difficulties, its cylindrical 

 shape being remarkably uniform. It has been shown in Chapter VI. 

 how lateral roots originate endogenously and in acropetal succession 

 on the main root. They repeat its characters, and these may be 

 repeated again in roots successively of higher order. However 

 complex the root-system may become, it is easily analysed, and all 

 its branches are held as equally of root-nature and homologous. 



The Shoot being both complex and variable in the relation of its 

 parts, presents many morphological problems. The fundamental 

 relations of its constituent parts, the axis and leaf, have been defined 

 in Chapter V., p. 69. The relations of the vegetative and floral 

 regions have been discussed in Chapter XIV., p. 277. Both are 

 referred in origin to the " general-purposes-shoot," from which they 

 are held to have been derived by a process of segregation. It may also 

 be held that scale and foliage leaves, bracts and bracteoles, and the 

 successive series of sepals, petals, stamens, and carpels are all types 

 of foliar appendage borne upon the axis. They are all so far homo- 

 logous that their relations to the axis which bears them are those 

 already defined : for they arise as lateral outgrowths from the 

 apical cone, springing from superficial and underlying tissues. They 

 appear in acropetal succession, and do not as a rule repeat the char- 

 acters of the axis which bears them. Their apparent differences 

 depend upon the fact that they are specialised in their development 

 to serve particular purposes in the plant. 



The shoot, thus composed of a simple axis and leaves, is the unit 

 from which even the most complex plant-bodies are built up. In 

 carrying this out, the shoot-unit may be multiplied in three ways : 

 (i) by axillary branching, (ii) by distal branching, and (iii) by adven- 

 titious buds. 



The first (i), that is axillary branching, is the common type for 

 Flowering Plants, a new apex being constituted in the angle between 

 leaf and stem, and the new shoot repeating the characters of the 

 original one (Figs. 5, 6). 



