APEX OF THE BOOT OF OSMUNDIA AND TODEA. 
85 
the neighbouring cells in point of size, still if the genetically 
connected groups of cells, enclosed between the darker marked 
anticlinal walls, be taken into consideration as products of 
successive segments, it is clear that the function of the apical 
cell is comparatively in abeyance, while the formation of new 
tissue is conducted with unusual activity by the cells derived 
from recent segments. Pyramidal cells, other than the apical 
cell, which appear at points removed from the organic centre 
(figs. 13 and 14, marked o ), do not seem to have any func- 
tional importance different from that of the cells immediately 
surrounding them. 
From the example shown in fig. 13 to that in fig. 16 the 
transition is easy. In the former case there are two rather 
irregular pyramidal cells of unequal size (x, x.), in the latter 
there are two such cells, of regular form and equal size, which 
overlap one another as seen in this section ; there is in the 
latter case a greater regularity in the subdivision of the seg- 
ments than is usually to be found in the roots of Osmund a. 
Lastly, the less common, but very interesting arrangement 
shown in figs. 17 and 18 must be mentioned. Here no pyra- 
midal cell is to be found; the median longitudinal section 
shows two cells (x, x.) of truncated pyramidal form, from which 
segments are cut off, (1) from the base, to form tissues of the 
root-cap; (2) from the sides, and, (3) as shown in fig. 17, from 
the truncated apex also. In this case the correspondence with 
SchwendeneFs description of the apical group in the root of the 
Marattiacese is very apparent. 
It has been demonstrated repeatedly by various authors that 
in the cases of Equisetum and many Ferns there is a certain 
order of succession and regularity of position of those walls by 
which the segments are divided up into smaller cells. A com- 
parison of the drawings above quoted, with those of Naegeli 
and Leitgeb, will suffice to show that no such regularity is 
found in Osmunda, even where a pyramidal cell is present; 
the continuity of the procambial tissue may be traced up to a 
point close to the apical cell or group of cells, those cells 
which are about to form tracheides being easily recognised at 
