20 



growth, only the meristem of the bundles having added a few small 

 thin- walled elements. 



It cannot be said that the thin-walled state of cells that normally 

 would be thick-walled is to be accounted for alone by keeping such 

 cells at a small size by confining the growth, since in all these 3 

 plants, these corresponding thin-walled elements are thick-walled in 

 the upper 8 mra or 10 mm of the segment within the cast, though there, 

 as below, the cells are no larger and the bundles no better developed. 



3. One plant that was much stronger than the foregoing had a 

 cast put about the 5th internode from the earth leaving one good- 

 sized internode exposed above the gypsum. The plant grew as well 

 as normal ones, reaching in 68 days a meter in height. At the end 

 of this period its structure was examined. 



Within the cast the preexisting cavity had been nearly filled by 

 the growth of tissues toward the centre. All kinds of elements showed 

 radial elongation. The hard bast was thinner-walled, and more thin- 

 walled derivatives from the cambium were present than without the 

 cast. The pith was better preserved within the cast. 



A general condition for all these Dahlia plants should be added : 

 The stem below the cast grows more slowly in thickness and produces 

 much less secondary tissue than above. 



Archangelica sativa Mill. 



The pith dies first in the second internode above the soil, the 

 cavity formation beginning schizogenously before elongation is com- 

 plete, the separation of cells being very small however before their 

 death ensues. "Within casts this plant makes good growth, no ab- 

 normalities appearing outside the gypsum. Three plants had upper 

 internodes put into casts, elongation not being complete in any ot 

 them, and probably no cavity existing. 



Twenty days after the application of the casts all 3 plants were 

 taken for examination. One had added 10 cm of new growth, the 2nd 

 15 cm and the 3rd nearly 20 cm , these differences corresponding to the 

 different strengths of the plants 20 days before. All showed a marked 

 constriction where the gypsum had surrounded them. 



In the plant of the weakest growth there was a very small cavity 

 present within the cast, one of about equal size above the cast and 

 a much larger one below the cast". Soon after the cast was removed 

 however the cavity within grew larger by the expansion of the com- 

 pressed stem. It is even doubtful whether a cavity had really existed 



