APEX OP THE ROOT IN OSMUNDA AND TODEA. 
83 
Longitudinal Sections. 
In organs having a single apical cell, from which segments 
of definite form are cut off in regular succession, the position 
and appearance of the apical cell itself, and of its segments 
when seen in longitudinal section, may be used as a test 
whether the longitudinal sections of the apex be accurately 
median or not ; but where there is no rigid regularity of form 
of the apical cell, or where there may be more than one initial 
cell present, the difficulty of judging whether a section be 
median or not is greatly increased. I have shown by means of 
transverse sections that the latter is the case in Osmunda; 
that where a single apical cell is present it is not always of 
uniform shape ; and, further, that the number of initial cells 
may be as high as three. Moreover, it' is not asserted that the 
varieties which may occur are by any means exhausted by the 
foregoing description. This being the case it is necessary to 
approach the study of longitudinal sections with great caution, 
and to subject the results to strict criticism. It is impossible 
to expect that there will be uniformity in the arrangement of 
the cells, as seen in longitudinal section, and the observations 
about to be detailed show that uniformity does not exist. As 
in the case of transverse sections, so also in observing longitu- 
dinal sections all the approximately median sections cut from 
one root were mounted and examined, usually on both sides, 
before conclusions were drawn. But beyond the mere diffi- 
culty of ascertaining the structure of the varieties of this irregu- 
lar meristem, there remains that of suggesting to which of the 
types of structure seen in transverse section any given one 
seen in longitudinal section most nearly corresponds. I do 
not profess to any near degree of certainty on this point, 
and where an opinion is put forward it must be understood 
that it is advanced only in a tentative manner. 
In the majority of cases cells of a pyramidal form have been 
found occupying an approximately central position in median 
sections; but in no single instance has an apical cell been 
observed having that regularity of form, and of arrangement 
