18 



will be found hollow in all the internodes except the lowest or the 

 2 lowest long before primary growth has ended. It is not unusual 

 however to find slender stems with 3 or 4 of their lower internodes 

 fully elongated and solid; and one stem was found where the cavity 

 showed first in the 7th internode above the soil, at least 5 lower 

 internodes having completed their elongation. This would seem to 

 be a clear case of normal preservation of the pith where the usual 

 rapid, radial expansion was not present. 



Cavity-formation is at first indicated in those internodes where 

 it comes into existence before elongation is complete by the inter- 

 cellular spaces growing at the middle of the pith into small clefts. 

 The collapse of cells then follows immediately. In those internodes 

 where the cavity first appears after secondary growth has begun, the 

 cells collapse without being separated by more than the usual inter- 

 cellular spaces at the angles. The two methods pass one into the other. 



Dahlia grows well or ill when surrounded with a cast according 

 to the age of the plant at the time of application. A young plant 

 stops its growth to begin it again after a rest lasting from one to 

 many weeks. One plant after the cast was applied grew slightly in 

 thickness and height for a few days, and then for 15 weeks made no 

 farther growth while a normal plant of same age and size growing 

 in the same pot reached the height of nearly a meter. At the end 

 of these 15 weeks the resting plant was placed where the sun could 

 shine directly on it from morning to night when growth again started 

 and proceeded normally. Above the upper limit of the cast there is 

 always found a swelling thicker than the normal stem, in which all 

 tissues show themselves increased, the outer covering being of cork. 



Five stems that were surrounded with casts have been examined. 



1 . The first of the series had 3 internodes above the earth when 

 the cast was applied, and the second internode was enclosed. In 

 the 30 days following, the plant grew r well in length adding 4 inter- 

 nodes, but the growth in diameter was only 2 1°/ of its former thickness. 

 Above and below the cast the bundles have grown in number of 

 elements, especially in the soft bast and in the thick-walled xylem 

 parenchyma. Moreover the cambium ring has been completed and 

 has given rise between the bundles to a band of thick-walled tissue 

 2 or 3 cells in radial width, while the outer pith- cells have become 

 thick-walled. 



Within the cast there have been fewer cells formed, the phloem 

 has grown but a little in both hard and soft bast, the vessels have 

 increased by 2 or 3 in a bundle and are much distorted, less than 

 half the number of thick- walled xylem parenchyma cells are present 



