15 



may continue for weeks. By the progressive death and collapse of 

 cells the separate cavities unite to form a hollow pith. 



In 5 young plants the 2nd internode above the soil was put into 

 a cast before any cavity was present in the pith and before elongation 

 had ended. The plants grew fairly well, and without distortion. 



Two plants that had doubled in size in the interval were exam- 

 ined after a period of 18 days. The pith was intact throughout the 

 plants. Above and below the casts the thick-walled cells in phloem 

 and xylem were beginning to appear. Within the casts there had 

 been less multiplication of cells from the cambium and no thicken- 

 ing of cell-walls. 



The 3 other plants were examined after growing within the casts 

 for 50 days. One had attained a height of 20 cm , one 35 cm and the 

 other 60 cm . The smallest plant showed below the cast a large cen- 

 tral cavity, above the cast several small cavities, and within the 

 cast a solid living pith. Above and below the cast many mechanical 

 cells showed in phloem and xylem , while in the cast only 3 or 4 

 bundles showed 3 or 4 thick-walled cells in each, some of these 

 thick-walled cells being hard bast and some xylem parenchyma. 



The plant of medium development had a large central cavity 

 below the cast, and several small cavities above and in the cast. 

 The mechanical tissue was within the cast much more strongly de- 

 veloped than in the preceding case, but still was of fewer and weaker 

 cells than in the same stem above and below the cast. Room for 

 the small widening of the bundle ring had been made by the com- 

 pression of the pith. 



The third plant and the one of the greatest development showed 

 small, isolated cavities above and below the cast, but still a greater 

 proportion of dead pith within the cast. The bundle ring here as 

 in the preceding cases was at a much smaller distance from the 

 centre than in normal parts of the stem. But in this individual the 

 bundle ring had within the cast nearly as many cells in a radial 

 row as in the same stem above and below the cast, averaging 40 

 outside the cast and 38 inside. The proportion of thick-walled cells 

 was however less, but not greatly less, within the cast than without. 

 This great increase in thickness of the vascular ring has taken place 

 at the expense of the pith which is greatly compressed. Because of 

 this movement of tissue toward the centre, the living pith cells and 

 the elements of the bundle, especially those of the xylem are much 

 elongated radially. 



In none of these plants does the cortex either in the normal or 

 in the abnormal parts of the stem have cavities. In none of them 



