THE STEM OR SHOOT. 



89 



above, a trichotomy in two dimensions would have 

 resulted. Nestler also found a stem of Amar a nthus sp. 

 which was trigonous instead of band-shaped. 



3. Ring-Fasciation. — There is another type of fascia- 

 tion which has been termed " ring-fasciation." The 

 shoots showing this phenomenon are less flattened and 

 more cylindric than is the case with those belonging to 

 the other types. In such a stem, which gradually 



Fig. 26. — Diagrams of transverse and longitudinal sections of a stem 

 to show construction of " ring-fasciation." The dotted line in the 

 latter shows the region of the transverse section ; and in the former, 

 where, theoretically, splitting should occur to cause bifurcation. 

 ep, epidermis ; p, pith. 



increases in diameter from below upwards, there ap- 

 pears in its upper portion a central cavity whose" 

 diameter increases upwards with that of the stem. 

 Arranged concentrically around the cavity are all the 

 various tissues of the stem in inversecl order, i. e. the 

 epidermis lines the cavity while the protoxylem of 

 the central cylinder lies face to face with that of the 

 normal cylinder of the stem ; hence all the tissues 

 of this peculiarly-constructed stem occur in two 



