29 



cells were especially slow in becoming thick-walled , the hard bast 

 appearing last of all; the cortex was always better preserved within 

 the cast than above or below, as was the pith. 



The changes found in this series and not occurring in the former 

 were those called forth by the presence of the cavity. The earliest 

 change within the cast is the enlargement of the cortical cells; and 

 since they cannot grow tangentially, they stretch out radially forcing 

 all cells which lie centrally from them inward and thus causing all 

 other cells to elongate radially. In this movement of cells the pre- 

 existing cavity is partly or wholly filled by the living pith- cells which 

 crush the remains of collapsed cells together in the centre. While 

 this movement of cells is taking place toward the centre the cambium 

 begins the formation of secondary tissue, more tardily than normally, 

 but earlier than in stems without cavity and in casts. The resulting 

 mechanical cells of the bundles are somewhat smaller than in the 

 normal stem and less numerous, but more mrmerous than in the stems 

 of the 1st series of plants. 



Melianthus major L. 



A young shoot of Melianthus major growing from the earth will 

 remain without cavity till it has reached a height of 4 to 6 inter- 

 nodes, the 2nd or 3rd internode being the first to become hollow. 

 When the cavity is once formed however it spreads rapidly upward 

 extending into the internodes still enclosed in the sheathing bases of 

 the terminal leaves. There is thus shown a great change in the 

 relation of the time of cavity-formation to the termination of elon- 

 gation. When the shoot growing from the earth is but a few inter- 

 nodes in height elongation has ended with the pith still solid in the 

 lower 2 or 3 internodes ; when the shoot has attained a greater height 

 the cavity is present long before elongation is complete, usually be- 

 fore the internode has attained l / l0 of its length. 



The beginning of cavity-formation is the separation of cells by 

 the growth of intercellular spaces in the middle of the pith. There 

 the cells of the central mass have more surface exposed to the inter- 

 cellular spaces than is in contact with neighboring cells. Soon after 

 a cell has all of one side thus exposed it dies and collapses. Nowhere 

 could dead cells be found that were not thus exposed, while small 

 rents could be seen to be bounded by living ceils. 



In the research on this plant 7 individuals had internodes enclosed 

 in plaster casts and 2 in opaque envelopes. The opaque wrappings 



