17 



Above and below the cast were splits in the pith and cavities 

 in the cortex. The elements derived from the cambium ring numbered 

 radially 2 more than within the constricted segment. Thick-walled 

 cells had made their appearance in the usual places, both above and 

 below the cast, — in the phloem, the xylem and in the cells limiting 

 the xylem centrally. At the upper limit of the cast the thick-walled 

 xylem parenchyma was greater in quantity than above or within 

 the cast. 



2. Examination after 40 days. Before this time 2 of the 5 plants 

 in casts had died and the remaining 3 showed various degrees of 

 development, none of them reaching more than 12 cm above the cast. 

 All of them showed within the cast some changes not present in the 

 plant examined after the interval of 23 days. 



Each of these 3 plants had a central cavity below the cast, none 

 within and only 2 of them a small cavity above. 



All of them showed a solid living cortex of radially elongated 

 cells within the cast, but cortical cavities above and below, with little 

 destruction of tissue and cells with radial elongation, which however 

 may have been caused by the room for growth afforded by the gypsum 

 not filling all the unevennesses of the surface of the stem. All of 

 them had within the cast a little mechanical tissue besides the col- 

 lenchyma. The first thick-walled cells to appear in the constricted 

 segments were the xylem parenchyma; in the stoutest stem bast fibres 

 and lignified cells bounding the bundles internally were also present. 

 Within the casts the thick-walled cells were in no case half so 

 numerous as in the same stem above and below the cast. The cells 

 which have become thick- walled since the last examination are evidently 

 not cells that have been formed since that time. The vessels in all 

 3 stems in the constricted segments are crumpled and smaller than 

 normal. The xylem parenchyma thick- walled cells are generally 

 smaller within the casts than without, but still they are no smaller 

 than the smallest outside the cast. 



One of these plants, the one that shows the greatest growth, has 

 at the upper limit of the cast more than the normal amount of thick- 

 walled xylem parenchyma. 



Dahlia variabilis W. 



The cavity appears in the pith of Dahlia at various ages of the 

 plant, the time of its beginning seeming to depend more on the 

 radial than the longitudinal extension. In the average stem the pith 



2 



