8o4 



A TEXTBOOK OF THEORETICAL BOTANY 



to define exactly. Protoxylem elements are always narrow, and because they 

 are formed in tissue which is actively elongating, they become greatly 

 stretched and are sometimes disrupted more or less completely by the growth 

 forces of the cells around them. Metaxylem matures only after elongation 

 of the tissues is complete and so is not subjected to stretching, though the 

 cells may grow apically and become very long. Protoxylem cells usually 

 have annular or spiral thickening and the later-formed metaxylem cells are 

 usually pitted, but there is no hard and fast boundary line to be drawn on 

 this character. It seems best to use the term protoxylem in a purely topo- 

 graphical sense, for those xylem elements, whatever their character, which 

 appear first and thus fix the pattern for the subsequent difi^erentiation of the 

 rest of the primary xylem. 



The phloem also may be differentiated into protophloem and meta- 

 phloem, the former having narrow sieve tubes which are usually soon 

 crushed out of existence. Here there is even less evidence of histological 

 distinction than in the xylem, and the terms are purely topographical. The 

 protophloem develops next to the pericycle, at the same time as the proto- 

 xylem, while the metaphloem is contemporaneous with the metaxylem. 



Branching of Roots. 



The normal mode of branching is monopodia!, the main root persisting 

 and giving off laterals. Very few exceptions are known among Angiosperms. 

 The tuberous roots of Orchids are often dichotomous or polytomous, and 

 mycorrhizal roots are almost always dichotomous, but these are the only 

 known cases. 



Among the Lycopodiales dichotomy of roots is the rule, and it was 

 probably also the case in the primitive axis from which both roots and shoots 

 have been evolved. There are grounds for holding that it is the most 

 primitive form of branching. It is interesting therefore to note the suggestion 

 of von Goebel that endogenous branching may have arisen by the modification 

 of a primitive dichotomy. The four sketches given in Fig. 798 are sufficient 

 explanation of the hypothesis. 



^ 



ft 



I 



Fig. 798. — Diagram to illustrate the theory of the origin of endogenous branching 

 from a primitive dichotomy. {After Goebel.) 



