STRUCTUEB AND DEVELOPMENT. 83 



usually cut off, and become elongated ; these lie 

 alongside the internodes of the primary series and 

 form fresh rows which are known as the secondary 

 cortical series. When these lateral (secondary) cells 

 are about the same length as the internodal cells of the 

 cortex, one being formed on each side of each nodal 

 cell, the result is the production of two complete rows 

 of secondary cells between each pair of primary rows, 

 and the cortex is then styled Triplostichods, there 

 being three rows of cortical-cells to each branchlet, 

 one primary and two secondary (Figs. 8 i, 9 i, ii). 

 In many species, however, the lateral (secondary) 

 cells are only about one half the length of the inter- 

 nodal cells, and by a process of accommodation, each 

 cell growing upwards or downwards, combine to form 

 a single row of cells between each pair of primary 

 rows. The cortex is then styled Diplostichous, there 

 being two rows of cortical-cells to each branchlet, one 

 primary and one secondary (Figs. 8 ii, 9 iii, iv). 



"When the secondary cortical-cells are truncate at 

 the ends and therefore do not overlap, the cortex is 

 easily seen to be either triplostichous or diplostichous, 

 but it frequently happens that intermediate states 

 occur, since the ends of the secondary cells often 

 taper and overlap the adjacent cells originating from 

 the adjoining primary row, thus producing a sub- 

 triplostichous condition, and a transverse section of 

 the stem will then disclose sometimes one and some- 

 times two secondary, between each pair of primary, 

 cells. 



In some species the cells of the secondary series 

 are of equal diameter and prominence with the inter- 

 nodal cells of the primary series, so that a transverse 

 section of the stem shows a number of tubes of equal 



VOL. I. 3 



