SECONDARY THICKENING IN MONOCOTYLEDONS. 



619 



been differentiated, and are situated 14-18 and 2o«'n, or many internodes below the 

 slowly elongating apex. The initial layer of the cambium is a layer of parenchyma- 

 tous cells characterised by no further peculiarities, which runs round the outer surface 

 of the bundle-cylinder, and is thus extrafascicular. It is in close proximity to the 

 outermost leaf-trace bundles, and must doubtless be regarded as belonging to the 

 plerome-cylinder. 



Radial growth of this layer and tangential divisions in reciprocal succession 

 produce in centrifugal order secondary wood, in centripetal direction secondary 

 cortex, in the same way and with similar 

 arrangement to the extrafascicular cam- 

 biums described on p.59i,thit \s,secondary 

 vascular bundles are formed, alternating 

 with interfascicular tissue, which is always 

 for the most part parenchymatous in the 

 plants under consideration. Comp. Fig. 

 241. The true initial layer remains 

 meanwhile always as a simple layer of 

 cells. Its cells, as well as those produced 

 from it, have the simple structure de- 

 scribed on p. 465 for cambium and young 

 secondary thickening. The form of both 

 is that of erect rectangular prisms, which 

 according to the individual case are 

 2-4 times as high as they are broad. 



Those sides of their rectangular bases, 

 which are shorter in a degree which varies 

 ■according to the stage of their develop- 

 ment, are radially directed. In accord- 

 ance with their mode of origin all the 

 cells are arranged in radial rows. Those 

 on the inner side of the initial layer 

 develope on the one hand in centrifugal 

 succession, and usually after one or more 

 tangential divisions, into permanent inter- 

 fascicular relatively wide parenchym.atous 

 cells. On the other hand, rapid longitudinal divisions facing in various directions 

 appear at definite points in tissue-iiiother-cells, or longitudinal rows of them, 

 separated internally from the cambium : from these are derived narrow-celled initial 

 strands, which develope into secondary vascular bundles; the further develop- 

 ment of the latter proceeds in centrifugal succession, while at the outer margin of 

 the strand a formation Of interfascicular parenchyma again begins. According to 

 Millardet, one to three as seen in transverse section, or as many as nine and twelve 

 original tissue-mother-cells, according to the size of the bundle, take part in the 

 formation of aft initial strand. When the number is large the cells concerned 

 always belong to several radial rows. The arrangement and succession of the 

 initial strands may .be concluded from the arrangement of the vascular bundles to be 



FIG. 241.— Piece of a transverse section of a stem of a Dra- 

 cxiia, probably D, reflexa, .about 1310111 thick and i™ higli, 

 slightly magnified, e epidermis ; /t periderm ; r primary cortex ; b 

 a bundle of the leaf-trace passing out through the latter ; fr pri- 

 mary bundles of the stem embedded in parenchyma ; m, x zone of 

 secondary growth and cambium with initial 'strands ; fcrther in is 

 mature wood; g secondary vascular bundles; st interfascicular 

 bands resembling medullary rays. From Sachs' Textbook. 



