210 MONOSTROMA, ULVA, AND NITELLA LESS. 



plane parallel to its base, into two cells, the upper (distal) of 

 which is the new apical cell (B, ap. c\ while the lower is now 

 distinguished as the sub-apical or segmenfal cell (s. ap. c). 

 The sub-apical cell divides again horizontally, forming two 

 cells, the uppermost of which (c, nd*) almost immediately 

 becomes divided by vertical planes into several cells (D, nd 4 ) ; 

 the lower (c, D, int. nd*} remains undivided. 



The sub-apical cell is the rudiment of an entire segment ; 

 the uppermost of the two cells into which it divides is the 

 rudiment of a node, the lower of an internode. The future 

 fate of the two is shown at once by the node dividing into 

 a horizontal plate of cells while the internode remains 

 unicellular. 



Soon the cells of the new node begin to send out short 

 blunt processes arranged in a whorl : these increase in size, 

 undergo division, and form leaves (A D, / 2 , / 3 ). 



These processes are continually being repeated ; the apical 

 cell is constantly producing new sub-apical cells, the sub- 

 apical cells dividing each into a nodal and an internodal 

 cell ; and the nodal cell dividing into a horizontal plate of 

 cells and giving off leaves, while the internodal cell remains 

 undivided. 



The special characters of the fully-formed parts of the 

 plant are due to the unequal growth of the new cells. The 

 nodal cells soon cease to grow and undergo but little altera- 

 tion (comp. ?id l and ;/^ 3 ), whereas the internodes increase 

 immensely in length, being quite 3,000 times as long when 

 full-grown as when first separated from the sub-apical cell. 

 The leaves also, at first mere blunt projections (A, / 2 ), soon 

 increase sufficiently in length to arch over the growing point 

 and so form the characteristic terminal bud : gradually they 

 open out and assume the normal position, their successors 

 of the next younger whorl having in the meantime developed 



