38 DOUGLAS HOUGHTON CAMPBELL ON THE 



fronds, altliongli becoming brown and dry, remain standing through the winter, owing to 

 the stout and firm stalk. In these dried fronds the pinnae often flatten out, and their 

 segments become sejjarated so as to faciUtate the dispersion of the spores. 



Sectiou V. The Stem. 



The gro'Nving point of the stem is completely concealed by the young leaves w^hich 

 meet over it, and in addition to this tlie numerous glandular scales interfere seriously 

 with its study. Unless care is taken, it is an easy matter to confound one of the 3'oung 

 leaves with the apex of the stem, for when seen from the side, l)eing conical in form and 

 having a definite apical cell, the resemblance is quite marked. The apex of the stem it- 

 self (PI. 7, fig. 1), when a section passes directly through it, is seen to be miich broader 

 than the young leaves. On account of the dense contents of the cells, the primary 

 meristem is difficiilt to study unless some method of clearing the tissues is employed. 

 The ordinary potassium hydrate solution will usually' answer, neutralizing when neces- 

 sary with acetic acid. 



The apical cell (fig. 1, «), whether seen from the side or from above, appears triangu- 

 lar, being, in fact, a tetrahedron. Segments are cut off fi'om the lateral fiices in regular 

 succession, the direction of the spiral being usually from left to right. Each segment is 

 first divided into two cells by a longitudinal wall at right angles to the lateral wall of 

 the apical cell. Each of these cells is next di'vdded by a transverse wall into an inner 

 and outer cell (fig. 1), and subsequently each outer cell is divided into two by a longi- 

 tudinal wall ])arallel to the lateral wall of the apical cell. Each oi-iginal segment is thus 

 divided into six cells, two inner and four outer ones. From the imier cells the plerome 

 cylinder is formed; from the outer the periblem and the dermatogen. This latter (figs. 

 1, 2, x) results from a series of walls formed in the outer cells parallel to their outer 

 walls. 



The inner cells of the segments increase rapidl\', the divisions being mostly longitu- 

 dinal, so that the lateral growth of the segment is much greater than the longitudinal, 

 and the diameter of the stem is rapidly enlarged, so that the end has the form of a cone ' 

 with broad base and ol^tusely angular apex. 



The outer cells are at first nari'ow, as seen from the side, being really tabular; later, 

 losing this by divisions perpendicular to the broader faces of the cells. 



The mass of meristem, resulting from these divisions, is composed of small, polyhedral 

 cells of about equal transA'erse and longitudinal diameters, filled with densely granular 

 protoplasm and provided with a distinct nucleus. Very early, a careful examination shows 

 a distinction betAveen the cells developed from the inner cells of the segments and the 

 outer ones. The former are rather longer and their length somewhat greater in propor- 

 tion to their breadth. These inner cells, which form the plerome in the stem of the 

 mature plant, now become separated into two parts : an inner one which retains for some 

 time its original character and an outer layer whose cells become di^aded by a series of 

 longitudinal Avails into a mass of long, narrow procambium cells. This procambimn cyl- 

 inder is not continuous, but is interrupted at intervals corresponding to the position of 

 the leaves, by spaces in which the cells resemble those of the inner part of the plerome 

 cylinder. These are the foliar gaps, and through them the inner part of the plerome, 



