84 



PLAXT JJFE. 



the primary stem. The stem quadrant, by repeated divisions, 

 quickly specializes a central cell, wliich becomes the apical 

 cell of the new shoot. Ordinarily it takes the form of a 

 three-sided pyramid, whose base forms the extreme tip of the 

 developing shoot {s, fig. 76, /, fig. 95). From the three inner 

 faces, as described for the root (^ 77), segments are constantly 

 formed, whose further divisions produce all the tissues which 

 constitute the members of the mature shoot, i.e., the stem 

 and the secondary leaves. In some fernworts and in the seed 

 plants, the posterior hemisphere resulting from the first divi- 

 sion of the egg grows into a filament called the suspensor, 

 and the jiriniary shoot develops from the anterior hemisphere. 

 (See fig. 80. ) In these plants ordinarily two or more cells 

 at the apex of the primary shoot are specialized as the initial 

 cells, and from their segmentation arise the tissues of the 

 whole shoot, as in the fernworts. 



Fir,. 95. — Median longitudinal section tlirr.ugh the apex of a shoot of the horsetail 

 {lu/if tSL'tuin ar^'^'iist:-), showing primary nieristeni and the fonn of the growing point. 

 i, apiral ceil, from which a segment, .S'. lias just been cut off by wall/. .S", a seg- 

 ment previously cut off, has divided by wall ///. /, /',/", successively older leaf 

 fundaments; i;, the initial cell of a bianch. Magnified 160 diani. — After Strasburger, 



101. Primary meristem. — Whether the tip of the shoot 

 be occupied by a single initial ixdl or by a group of initials, 

 the ajiical region, in which the formation of new cells is 



