-L-jo THE STRUCTURE OF ECONOMIC PLANTS 



undulating zone of cambium which connects some of the bundles 

 directly, and passes outside of others, and the latter may be sepa- 

 rated from the subtending cambium by several layers of paren- 

 chymatous cells. 



As the activity of the anomalous interfascicular cambium pro- 

 ceeds, a secondary cambium arises in the pericycle. This at first 

 produces only xylem elements and parenchyma by centripetal 

 divisions; but, later, it forms groups of phloem and parenchyma- 

 tous cells centrifugally. Finally, at the points of phloem forma- 

 tion, the secondary cambium becomes inactive and is itself differ- 

 entiated into vascular tissue and parenchyma. This results in a 

 discontinuity of the extrafascicular or pericyclic cambium, but 

 new segments of secondary cambium appear on the outer face of the 

 extrafascicular phloem which become confluent with the older 

 persistent segments of pericyclic cambium, thus restoring the con- 

 tinuity of the cambial ring. This cambium may then undergo 

 reciprocal divisions producing xylem and phloem tissue. 



As a result of this anomalous type of secondary thickening, sev- 

 eral systems of mechanical tissue are laid down, (i) The periph- 

 eral portions of the primary bundles are bounded by a zone of 

 thick-walled cells in which scattered groups of phloem are located. 

 The cells are not true xylem, except at points immediately centrad 

 to the phloem groups, but form connective or conjunctive tissue 

 consisting of cells which may be parenchymatous or prosenchyma- 

 tous. (i) The extrafascicular or secondary cambium forms an 

 irregular series of vascular bundles which are surrounded by thick- 

 walled mechanical tissue. (3) The outer zone of the pericycle, 

 from which the secondary cambium originates, is delimited by 

 sectors of lignified fibers. (Fig. 134.) 



In summary, there are three phases of cambial activity with re- 

 spect to the secondary and tertiary thickening of the floral axis 

 and vegetative stem: (i) the production of a normal fascicular 

 cambium which produces phloem and xylem in the usual manner; 

 (2.) the formation of an interfascicular cambium which forms 

 unilateral connections with the fascicular cambium; and (3) the 

 development of a .secondary or pericyclic cambium, sometimes re- 

 ferred to as extrafascicular, which produces both xylem and phloem 

 elements. Xylem and parenchyma are first produced centripetally 

 by the pericyclic cambium, followed by the centrifugal formation 

 of phloem and parenchyma. Ultimately, sectors of the extra- 



