XIII 



EQUISETINEyE 



451 



circles about the upper part of the axis of the fertile shoots 

 (Figs. 227, 242). 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 summit. Circular foliar sheaths are formed in the 

 same way, but the leaves form rounded elevations, either 

 entirely separated or but slightly joined (Fig. 237). These 

 are at first nearly hemispherical, but soon become constricted 

 at the base, and about the same time the first trace of the 

 sporangia can be seen. A section of the young sporophyll 



rv 



Fig. 237. — A, Longitudinal section of the apex of a young fertile shoot, x i6 ; B, apex of the same, 

 X 160 ; s/i, young sporangiophore ; .i', apical cell. 



shows that the centre of the prominence 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 sporangium 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 

 BotrycJiiuni. The outer cell divides again. The innermost 



