MERISTEM. 



53 



"open" bundles — viz., bundles capable of receiving fresh annual 

 rings of xylem and phloem by the activity of the cambial layer. 

 In a few instances, however (JDraccena), the stems of certain 

 Monocotyledons possess zones of meristem from which fresh 

 annual rings of fibro-vascular bundles are produced, these bundles 

 being, however, always of the closed variety ; and it may be here 

 mentioned that petioles of bifacial leaves in Dicotyledons possess 

 only scattered closed bundles, there being no cambial layer in 



TTVCL 



Fig. 36. — Portion of a Transverse Section through the Stem of 

 Ricinus communis. — .i-. Cambium ; ph, phloem ; <cy, xylem (one large 

 vessel is seen amongst the traoheides) ; md, medullary ray. 



each bundle, and thus no possibility of secondary thickening. 

 In centric leaves {Pinus), on the other hand, where a central 

 cylinder is present, there may be, for a short time, a narrow 

 zone of cambium between the xylem and phloem portions of 

 the fibro - vascular bundles. In some dicotyledonous stems 

 (Podophyllum peltatum) scattered vascular bundles occur instead 

 of a well-defined ring of wood and bast; this is known as 

 anomalous stem-structure. Other instances also occur. 



