7H 



A TEXTBOOK OF THEORETICAL BOTANY 



producing a corky bark which replaces the outer armour of leaf bases and 

 finally forms the stem surface in the lower part of the oldest plants. In 

 some other species the leaf bases are never lost and the age of the plant may 

 then be gauged by counting them. 



Secondary thickening is not vigorous. In Zamia, Dioon, Stangeria and 

 Ceratozamia there is only one ring of vascular bundles, which slowly increase 

 in size radially by means of cambium. As the bundles increase in size the 

 medullary rays between them become so narrowed that the wood appears to 

 form a continuous ring. In Dioon the wood may show rings like annual 





LtQ^ trace girdle 



Vascular ring m^j 





Fig. 717. — Zamia floridana. Transverse section of young 

 stem showing large cortex containing girdle bundles and 

 small vascular ring with numerous medullary rays. 



rings, which are, however, formed at irregular intervals. Only in rare cases 

 does the zone of secondary wood reach any considerable thickness. This is 

 called the monoxylic type of stem. Cycas, Macrosamia, Encephalartos and 

 Bowenia start by being monoxylic but later develop several successive vascular 

 rings, concentric with the first, and are therefore polyxylic (Fig. 718). The 

 cambium of the first ring remains active for only a short time and is succeeded 

 by another which is formed independently in the pericycle or cortex. This 

 again is superseded by a third. As many as fourteen rings have been found. 

 Usually the xylem and phloem of each ring are normally orientated. In 

 Macrozamia, however, a tertiary ring with inverted orientation may appear 

 between the first and second rings, and in Cycas occasional arcs of inverted 

 bundles may occur between the normal rings. This may indicate a partial 

 reversion to a structure with concentric bundles and has been held to indicate 



