33 



plant where secondary growth has not begun and where elongation 

 is still progressing, the intercellular spaces before any cells die grow 

 so large as to divide the wall completely, but the space thus formed 

 has never been found wider than the diameter of a cell or two be- 

 fore the collapse of surrounding cells begins. In the hypocotyl the 

 cells collapse before any intercellular spaces have grown into clefts. 

 In neither hypocotyl nor the parts above have dead uncollapsed cells 

 been found. Thus, from the nature of the case, the cavity in the 

 hypocotyl is formed only from the collapse of cells, while in other 

 parts of the plant it is formed largely from the movement of the outer 

 living pith-cells from the centre during the normal expansion of the 

 stem. 



Ricinus communis for months suffers very little from the appli- 

 cation of casts. The constriction may be as small as 1 / 2 the diameter 

 of the stem above and below, but the plant seems to develop to the 

 height of a foot or more just about as well as normal ones. How 

 early this very narrow piece of the stem within the cast begins to 

 show its effect on the development of the whole plant has not been 

 determined. 



The irritation given by the cast shows itself however at the 

 upper and lower surfaces of the gypsum-cylinder. Above the cast 

 the stem swells a little abnormally and gives rise to several lateral 

 roots. Sections through this part show all tissues very active in cell- 

 division. Below the cast the same effect is produced only much 

 weaker in degree. For a centimeter or two above the cast the for- 

 mation of cavity may not occur, and the whole segment of stem 

 below the cast does not enlarge so rapidly as normally. 



Two series of experiments have been made with Ricinus, one 

 with lateral shoots from the stem and the other with young stems 

 grown from seed. 



1. For the first series the newly-formed branches on stems of 

 one season's growth that had been in the autumn cut back and placed 

 in pots in the plant-house were used. These branches often grew to 

 a length of 3 or 4 inches including 5 or 6 internodes before any 

 cavity appeared and hence were favorable for the study of that kind 

 of cavity- formation which appears before elongation has ceased. More 

 than a dozen of these branches were surrounded with gypsum, but 

 they as well as the normal ones made but poor growth, and many 

 of them died because the main stems bearing them did. Four grew 

 sufficiently to give some results, adding after the casts were applied 

 from 2 to 5 internodes. These branches all showed a better preser- 

 vation of the pith within than outside the casts, and 3 of them showed 



3 



