VI] INITIAL CELLS OF STEMS 107 



the growing point: i.e. the apical cell is a spot in the embryonic tissue in 

 which neither anticlines nor periclines nor radial longitudinal walls have yet 

 been formed." So far then from the apical cell dominating the apex its 

 characters are negative. What we shall see is that the degree of negativity 

 of this cellular construction of the apex as expressed in the absence of certain 

 walls provides a basis for comparative treatment, related as this is found to 

 be to the bulk of construction of the organisms compared. 



The most delicate and least bulky in construction of the Filicales are 

 those which have been distinguished as the Leptosporangiate Ferns. In these 

 it has been recognised since the time of Naegeli and Leitgeb(86) that a single 

 initial cell is present at" the apex of stem, leaf, and root. This is, however, 

 by no means uniform for all the Filicales, as the Eusporangiate Ferns show. 

 It is in point of fact an extreme condition. Nevertheless for clearness of 

 exposition it will be taken first, as being the most definite type of structure 

 of them all, as it is also the most generally known. 



The axis of most Leptosporangiate Ferns is terminated by a slightly 

 conical tip, at the distal centre of which is a single apical or initial cell. In 

 most cases this cell has the shape of a three-sided pyramid, of which the base 

 forms part of the outer surface of the plant, while its apex is directed inwards. 

 From the three slightly convex sides segments are cut off in regular succession 

 by walls parallel to them. Consequently the fourth segment will be opposite 

 the first, the fifth opposite the second, and so on (Fig. 100). It is stated that 

 in such a rhizome as that of Polypodiwii vulgare only a few such segments 

 are formed in each year(75). The stem of the Bracken is exceptional in the 

 fact that the initial cell may be only two-sided. It 

 is shaped like half of a biconvex lens, and is set 

 on edge in the creeping rhizome, with its segments 

 coming off alternately right and left (Fig. 100, A). K 

 vertical section in either case discloses the obconical 

 form of the initial cell, and the relation of it to its 

 segments is shown in Fig. loi, for TricJwmanes, in 

 which the cell has the usual three-sided form. 



r^. , r • •,• 1 11 1 , -,1 .1 Fig. loi. Median vertical sec- 



The same shape of mitial cell, but with the fion of the apex of the stem 

 additional complication of segments cut off from of Trichomanes radicans. 

 its base to form the root-cap, is found in the roots 



of Leptosporangiate Ferns. A transverse section traversing the apical cell 

 and its latest segments which go to form the body of the root, shows the 

 scheme in ground plan (Fig. 102, B). The segments are arranged in three 

 rows corresponding to the three sides of the apical cell, and are separated 

 by zig-zag lines, which are called Xkv^ principal walls {p, p). In transverse 

 sections below the apical cell these principal walls can still be traced : but 

 a notable additional feature is seen in the presence of sextant walls {s, s), 



