35 



nowhere present. In each stem cortical cells had died within and 

 without the cast, and within the cast the more vigorous vascular zone 

 had crowded the weak cortical cells so that a collapsed hand coursed 

 through the middle of the cortex. Without the cast the cortex, made 

 very loose hy the death of some cells and by the great intercellular 

 spaces , retained its extended form. Thus within the cast the grow- 

 ing cells of the vascular ring made room for themselves hy crowding 

 together the weak cortical cells. 



The pith inside and outside the casts had the same approximate 

 diameter, the casts having been applied after primary extension had 

 ended, and the power of growth not being sufficient to force inward 

 the pith and the few secondary mechanical elements that may have 

 been formed before the effect of the cast had made itself felt. 



The effect of the pressure in retarding multiplication of cells was 

 marked, since within the casts, the number of xylem elements derived 

 from the cambium averaged in the 4 plants 1 / 2 the number above and 

 below the casts in the same stems. Thus within the casts the secondary 

 elements numbered in a radial row in the various stems from 18 to 33, 

 outside the casts from 30 to 65. The number of phloem cells derived 

 from the cambium was proportionally reduced by the cast. 



The xylem parenchyma cells showed much unevenness in thick- 

 ness of wall, and were generally thinner-walled within than without 

 the casts. They had a shorter radial diameter than usual, making 

 them wider tangentially than radially, the normal relations of these 

 2 diameters being reversed. The vessels suffered the same shortening 

 of the radial width. No change in the relative number of vessels 

 could be detected. 



The elements of the hard bast were everywhere fewer in number 

 and provided w T ith thinner walls than in normal positions. 



It is worth noting too that though the cortical cells within the 

 cast have been able to reach not much greater than half their normal 

 size, the cells of the bundle show a proportionally much greater size. 



In the plant about whose epicotyl a cast was placed before pri- 

 mary extension had ended and where a cavity was already present, 

 the only point worth noting that differs from what has been yielded by 

 the foregoing plants is the movement of all kinds of cells toward the 

 centre, pith, fibrovascular and cortical elements showing radial elongation. 



3. Envelopes of black cloth have been wrapped around lateral 

 branches like those experimented with in the 1st series and around 

 stems of young plants like those of the 2nd series, and though perfect 

 etiolation has been secured none of the marked changes ascribed to 

 the effect of casts have been found. 



3* 



