SECT. I 



MORPHOLOGY 



103 



organised i)laiits. The apicul cell of the common Horse-tail {Equiselwiii 

 arvense) will serve as an example (Figs. 113, 114). The apical cell of 

 the main axis viewed from above (Fig. 1 14 yi) appears as an equilateral 

 triangle, in which new walls are successively formed in a spiral direction, 

 parallel to the original walls (/>). Each new segment thus derived is 

 divided by anew division wall (Figs. 113, 114 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. 114 i>), becomes again divided 

 by a sextant wall (s) into two new cells. It is unnecessary to trace the 



Fio. 113.— llediaii longitudinal section of tlie vegetative cone of Equisetum arvense. 

 Explanation in the text, (x 240.) 



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, Avliile 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 the formation of cell walls inclined alternately 

 inwards and outwards in the wedge-shaped marginal cells (Fig. 113/). 

 This is succeeded at a lower level by other and older leaf-whorls (/',/")• 

 An initial cell (g) may be distinguished in the axil of the second leaf- 

 whorl, and this is destined to become the three-sided apical cell of a 

 side branch. 



