16 



are the cortical cells noticably elongated in the radial direction except 

 in those places where the elongation can be accounted for by growth 

 outwardly, where the gypsum by a leaf base or other means was kept 

 from confining the expansion. 



In all these plants there was an excessive development of thick- 

 walled cells in phloem and xylem at the upper limit of the casts. 



Urtica dioica L. 



In Urtica dioica the time and place of the first appearance of 

 the cavity is very diverse. A young thick stem that is growing well 

 will first show a cavity when a height of 6 or 7 internodes above 

 the earth has been reached, the 3rd or 4th internode being the first 

 to become hollow. A stem may attain a development of 12 inter- 

 nodes above the soil and still be completely solid. The cavity may 

 appear either before or after elongation has ceased ; it has never been 

 found nearer than 3 or 4 internodes from the apex of the stem. 

 These observations show that the cambium-zone may or may not be 

 complete when the cavity shows itself. 



Before a general cavity is present several schizogenous clefts are 

 found in the pith which are isolated longitudinally and transverely. 

 By the union of these clefts the general cavity arises and then enlarges 

 schizogenously and lysigenously. 



In the experiments on this plant 6 casts were applied around 

 the 2nd and 3rd exposed internodes from the apex. Cavities certainly 

 were not present in these internodes at the time, and the cambium- 

 zone was not complete. The casts at first affected the growth of the 

 plants but little. There were no swellings nor distortions; and had 

 the succeeding weeks not been very dry the growth would probably 

 have been much greater. 



1. Examination after 23 days. This plant had grown nearly as 

 well as the normal ones by its side, having added above the cast 4 

 internodes with a total length of 9 cm . The cavity had extended no 

 farther within this period, the pith being solid from below the cast 

 to the apex of the stem. Growth had taken place within the cast, 

 since the cambium-zone was complete, and the ring of secondary 

 tissue contained 6 radial elements. Except the collenchyma, which 

 was present before the experiment began, no thick-walled cells were 

 to be found within the limits of the gypsum. Within the same 

 segment the preexisting cortical and pith intercellular spaces had been 

 closed up. 



