156 



BOTANY 



PART I 



provided with such apical cells is in reality due to the early develop- 

 ment of new apical cells in young segments (Fig. 163 b). In the 

 case of the erect radially symmetrical stems of the Musci, most Ferns 

 and Equisetaceae, the apical cell has generally the shape of an inverted 

 three-sided pyramid ( 148 ) with a convex base, and forms the apex of 

 the vegetative cone characteristic of the more highly organised 

 plants. In the common Horse-tail (Equisetum arvense), for example, 

 the apical cell of the main axis viewed from above (Fig. 165 A), 

 appears as an equilateral triangle, in which new walls are successively 



Kiii. 104. Median longitudinal section of the vegetative cone of Kyuisetvm arvense. 

 Explanation in the text. (X 240.) 



formed in a spiral direction, parallel to the original walls (p). Each 

 new segment thus derived is divided by a new division wall (Figs. 

 164, 165 m) into an upper and lower half; each of these halves, as 

 is shown most clearly by an optical section just below the apical cell 

 (Fig. 165 B), becomes again divided by a sextant wall (s) into two 

 new cells. It is unnecessary to trace the further divisions, and it 

 will suffice to call attention to the fact, that all cell walls parallel to 

 the outer surface of such vegetative cones or portions of plants are 

 termed PERICLINAL WALLS, while such as meet the surface and the 

 periclinal walls at right angles are designated ANTICLINAL; of the latter 

 those in the plane of the axis of an organ are called RADIAL. Some 

 distance below the apical cell of Equisetum arvense the first leaf-whorl 

 arises from the vegetative cone as a circular ridge, which grows by 



