1.64 THE STRUCTURE OF ECONOMIC PLANTS 



The primary phloem keeps pace with the stelar growth by con- 

 tinued divisions of the parenchyma, and the effect of this is to 

 scatter and stretch the sieve tubes until they are finally obliterated. 

 The pericycle also persists, undergoing continued radial divisions 

 of its cells; and, by the time the epidermis and cortical regions 

 are ruptured, it is several-layered as a result of successive periclinal 

 divisions. In the outermost of these layers, periderm formation 

 is initiated at about the time that the first secondary cambiums 

 , are beginning to form, this coinciding roughly with cortical dis- 

 integration. The phellogen cuts off phellem or cork cells centrif- 

 ugally and phellodermal cells centripetally, but the divisions do not 

 take place in regular alternation and more cork cells are cut off than 

 phellodermal cells. The outer cork cells are shed continuously 

 as the axis enlarges so that the phellem maintains a relatively 

 constant thickness of from five to eight cell layers. The cork 

 cells are flattened and the thin walls are suberized except for the 

 lignified middle lamella. 



The Secondary Cambium. — The first of the rings ^ of 

 the fleshy axis is formed by the activity of the primary cam- 

 bium. The remaining rings are produced by supernumerary or 

 secondary cambiums, the first of which arises while the 

 formation of the first ring by the primary cambium is still in 

 progress. 



Tertiary Thickening. — The origin of the supernumerary 

 cambiums and the mechanism of secondary and tertiary thickening 

 has been described by van Tieghem (19), de Bary (5), and Art- 

 schwager (i). According to the latter, the point of origin of these 

 cambiums varies with the level of the axis concerned. In the 

 root and lower hypocotyl, the first supernumerary cambium 

 originates in a zone of primary phloem parenchyma between the 

 pericycle and the secondary phloem. (Fig. 130.) In the upper 

 hypocotyl, it arises from the pericycle; and in the intermediate 

 region, from either the pericycle or phloem parenchyma. Outside 

 the protoxylem points, all secondary cambiums are of pericyclic 

 origin. The formation of additional cambiums then takes place 

 in the following manner: 



^ The use of the term ring in the description of the secondary and tertiary thickening of 

 the fleshy axis in this and succeeding sections is one of convenience and applies to the tran- 

 sectional aspect of the axis. Actually, each ring should be regarded as a conical layer of 

 tissue; and the secondary and tertiary rings of the axis as consisting of a series of successively 

 larger cones each of which encloses the one centrad to it. 



