31 



centre. The radial elongation was not so great. The number of 

 cells arising from the cambium has been reduced by the pressure of 

 the cast; since below the constriction the secondary xylem elements 

 number 26, within 16 and above 14, whereas the proportion would 

 normally be about 26 : 20 : 14. The prevention of growth in the 

 thickness of cell-wall is marked. Within the cast the zone of thin- 

 walled secondary elements lying centrally from the cambium is 2 cells 

 greater in width than outside the cast. The number of thick-walled 

 secondary xylem cells in a radial row is below the cast 24, within 12, 

 above 12, — the intermediate segment not showing as it would nor- 

 mally an intermediate number of such cells. Moreover within the 

 cast all of these xylem cells are considerably thinner-walled than 

 above and below. Apparently the cast has affected no change in the 

 number of phloem cells relative to the xylem cells. Above and below 

 the cast is a wide zone of outer pith-cells thick-walled and lignifled. 

 Within, these cells have preserved their weak condition. 



A new tissue has appeared in this shoot, — the phellogen lying 

 just externally to the starch-sheath. Above and below the cast this 

 zone is from 3 to 5 cells in width, and the cortex is beginning to 

 break down. Within the cast the phellogen has not completed the 

 circuit of the stem and the cortical cells are all intact. 



3. The remaining 5 shoots have produced since the casts were 

 applied respectively 3, 4, 5, 8 and 10 internodes in periods extending 

 from 59 to 1ST days. All of them show cavity throughout, but the 

 pith is always better preserved within the cast than outside of it. 

 All of them show radial elongation of pith-cells and of the oldest 

 xylem cells. In all, the number of mechanical cells in xylem and pith 

 is within the cast in excess, of that above and below the cast. Inside 

 and outside the limits of the casts the phellogen is everywhere pres- 

 ent and the cortex outside of it is dead and shriveled. The secon- 

 dary phloem has shared with the xylem this increase of cells within 

 the casts. 



Normally the size of the cross-section of thick-walled xylem cells 

 varies greatly, the first formed being usually the larger. The size 

 of these cells within the cast showed no important variation from 

 the normal. The xylem cells that were formed before the cortex 

 died have not attained their normal thickness of wall. They can be 

 distinguished from those later formed by their radial elongation and 

 thinness of wall. The total amount of mechanical tissue within the 

 cast becomes greater than above and below, especially in the xylem. 

 The 2 shoots that had grown the most after application of the casts 

 showed the one over 40%, the other over 50°/ more tracheids within 



