xxi APICAL GROWTH 211 



sected away, the node from which they spring (nd l ) is found 

 to give rise distally to a very short internode (inf. nd z }, 

 above which is a node (nd 2 } giving rise to a whorl of very 

 small leaves (/ 2 ), also curved inwards so as to form a bud. 

 Within these is found another segment consisting of a still 

 smaller internode (int. nd^} and node, bearing a whorl of 

 extremely small leaves (/ 3 ), and within these again a segment 

 so small that its parts (int. nd*, / 4 ) are only visible under 

 the microscope. The minute blunt projections (/ 4 ), which 

 are the leaves of this whorl, surround a blunt, hemispherical 

 projection (gr. pf), the actual distal extremity of the plant 

 the growing point or punctum vegetationis. 



The structure of the growing point and the mode of 

 growth of the whole plant is readily made out by examining 

 vertical sections of the terminal bud in numerous specimens 

 (Fig. 46). 



The growing point is formed of a single cell, the apical 

 cell (A, ap. c\ approximately hemispherical in form and about 

 -5^5- mm. in diameter. Its cell-wall is thick, and its cell-body 

 formed of dense granular protoplasm containing a large 

 rounded nucleus (;/?/) but no vacuole. 



In the living plant the apical cell is continually undergoing 

 binary fission. It divides along a horizontal plane, i.e., a 

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

 which is the new apical cell (B, ap. <r), while the lower is now 

 distinguished as the sub-apical or segmental 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^}\ 

 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 



P 2 



