22 STRUCTURE AND LIFE-HISTORY OF HAY-SCENTED FERN. 



protuberance standing* out in the center of this basin is the growing point. 

 Its surface is naked and is rendered irregular by leaf -rudiments. At the 

 top of the protuberance is the stem-initial a narrow, irregularly triangular 

 cell. In longitudinal section it is deeper (0.0642 mm. to 0.07 mm.) than 

 any other cell, excepting a newly formed segment (figs. 106, 129). But 

 in cross-section it is smaller than many of its near neighbors (figs. 71, 103, 

 109). This narrowness makes it difficult to recognize. Further, as the 

 protuberance in which it stands varies in apparent position according to 

 the development of the young leaves, one can not be sure of getting satis- 

 factory sections of it by cutting a bit of stem in an exactly transverse or 

 longitudinal plane. 



Segments are cut off in regular succession on the three interior faces of 

 the initial. As I could not, even with much effort, distinguish any regular 

 position of the initial with regard to dorsal and ventral surfaces of the 

 stem, we can not speak of dorsal or ventral segments. The irregular 

 arrangement of the leaves is probably related to this irregularity of the 

 position of the stem-initial. The order in which the segments are cut off 

 is either from left to right or right to left, the two occurring in about equal 

 numbers in my preparations (7 counter-clockwise, 5 clockwise, from older 

 to younger). 



Each segment is divided first by a periclinal wall near its inner end. 

 The small, deep-lying cell thus formed gives rise to the medulla (fig. 106). 

 The outer cell is next cut by the sextant wall a radial anticline, dividing 

 the cell nearly into halves ("sextants"). This wall remains prominent 

 for a comparatively long time (figs. 71-75, 103, 109) (</. Bower's figures, 

 1889). The relative sequence of the following anticlines and periclines 

 was not determined. The second periclinal wall is formed near the inner 

 end of the columnar partial segment or sextant, giving an inner nearly 

 cubical cell and an outer columnar one (fig. 106). The inner of these 

 (plerome rudiment) gives rise to the vascular system of the stem, including 

 outer and inner endodermis. The sextants are usually halved longitudi- 

 nally at right angles to the sextant wall (fig. 73), and as many as sixteen 

 to twenty rectangular cells may be formed by walls at right angles to the 

 two just mentioned (figs. 75, 103). Very often oblique walls break up the 

 symmetry. The outer cells are repeatedly divided by periclines near the 

 inner end (fig. 106) until, after four or five such partitions, the remaining 

 outer portion is reduced to the depth of the epidermis . Division then ceases . 

 Meanwhile the whole segment has been pushed farther and farther from the 

 growing point of the stem. The last pericline occurs in cells which are 

 nearly half-way up on the sides of the basin-like depression of the stem apex. 



Near the lowest part of this depression arise the hairs which clothe the 

 apex (fig. 106). A superficial cell bulges out slightly and is cut obliquely. 

 The outer member enlarges in length and diameter, and is divided by 

 several septa. These divisions are often intercalary. The two basal cells 



