APICAL CELL OF EQUISETUM. 215 



cells, the dermatogen dividing only anticlinally, but clearly giving 

 rise to epidermis only, while it is equally easy to see that the 

 cortex arises from the periblem. 



In place of Euonymus we can use developing shoots of most 

 other trees or shrubs which have decussate leaves. The Ash, 

 Fraxinus excelsior, serves admirably. 



Growing Apex of Equisetum. -We will now study by means 

 of one of the Vascular Cryptogams a case in which, instead of 

 an apical meristem, as above, growth takes place by means of an 

 apical cell, from which segments are cut off, to the subsequent 

 division and differentiation of which all the tissues owe their 

 origin. We will select as the most favourable object Equisetum 

 arvense, the common horse-tail of cultivated fields. It is compara- 

 tively easy in this case to bring the apical cell into view. A 

 preliminary view of the nature of the apical cell, and its relations 

 with the segments cut off from it, may be obtained by a cross- 

 section which includes the apical cell itself. This is seen in Fig. 

 84 A. The apical cell has an equilateral triangular base, its sides 

 being somewhat convex, since the three angles 

 tend to be right angles, in accordance with 

 the general rule that a wall, when first formed, 

 tends to form right angles with the walls 

 which it intersects. Parallel with its side- 

 walls are formed ne\v dividing- walls, which cut 

 off flat segments successively from the sides 

 of the apical cell in regular spiral sequence. 

 Each segment subsequently undergoes division 

 by walls at right angles to that first formed. 

 (Fig. 84 A). 



For further purposes, shoots in course of 

 development can be studied either fresh or 

 preserved in alcohol. We remove from the 

 apex of the stem a piece about $ inch long, or 



rather more, and cut it between the fingers, of Equisetum 



.,1,1 ,, /?, optical cross-section 



with the apex downwards, or by the needle O f the same, tbcussed 



method already described. Amongst the loncji- der the apical cell. 

 tudinal sections produced we look for one which shows the conical 

 growing apex intact. In order to obtain an insight into the arrange- 

 ment of the cells of this apex, it is usually necessary to make them 

 more transparent. This may once more be effected with Edit de 



