XII 



EQUISETINB^ 



473 



circles about the upper i)art of the axis of the fertile shoots 

 (Figs. 266, 281). A section through the apex of the young 

 shoot shows much the same structure as a sterile one, but the 

 apical cell is smaller and the leaves do not arise so near the sum- 

 mit. Circular foliar sheaths are formed in the same way, but 

 the leaves form rounded elevations, either entirely separated or 

 but slightly joined (Fig. 276). These are at first nearly hemi- 

 spherical, but soon become constricted at the base, and about the 

 same time the first trace of the sporangia can be seen. A sec- 

 tion of the young sporophyll shows that the centre of the promi- 



FiG. 2j6. — A, Longitudinal section of the apex of a young fertile shoot, Xi6; B, apex 

 of the same, Xi6o; sp, young sporangiophore; x, apical cell. 



nence already has formed the young plerome which, as in the 

 ordinary leaves, joins that of the internode beneath. Just above 

 the base a cell may sometimes be detected, which is larger than 

 its fellows, and has a larger nucleus. From a comparison with 

 slightly older stages there is no doubt that this is the sporan- 

 gium mother cell, or more correctly the axial sporangial cell, as 

 the adjacent tissue also takes part in its further growth. This 

 axial cell now becomes separated into an inner and outer cell, 

 as in Botrychimn. The outer cell divides again. The inner- 



