176 SEGMENTATION 
the last century into this channel. Without their having any clear under- 
standing what the apical segmentation signified, it became an object for 
investigators to define its details in representatives of all the main groups 
of plants, and’ the attempt was made to correlate the segmentation observed 
with the initiation of definite external parts or internal tracts of tissue. 
With regard to the former, it is undoubtedly the fact that the appendages 
in certain of the lower organisms may be directly correlated with apical 
segmentation: this is seen in many Algae: in Mosses also each segment 
of the apical cell gives rise to a leaf, and Naegeli, who looked upon the 
apex as a dominating influence, held that the same was the case in 
Pteridophytes as well. But a general revision of the question has led 
Schwendener to the conclusion that the origin of the appendages in the 
Pteridophytes is not necessarily connected with or determined by the 
A 
Fic. gt. 
A=Apex of Eguisetum scirpoides : the swelling below shows the highest leaf-sheath : this 
extends upwards on the right to the segm.ent-wall, on the left only to the middle of a segment. 
&=trans. sec. of the same apex : the dotted line indicates the apical cell ; focussing downwards, 
the outline shows the youngest sheath, with its three leaf-teeth already indicated by the faintly 
three-lobed outline. /4/=principal walls. S=sextant walls; the position of the leaves is 
independent of these. x 550. (After Schwendener.) 
segmentation at the apex. The genus Egwu/sefum is a good case in 
point; for though the segments of the apical cell are constantly arranged 
in three longitudinal rows, still the number three does not dominate the 
variable numbers of leaf-teeth in the whorls of the mature plants of the 
genus. Moreover, as the leaves of successive whorls alternate, while the 
successive segments do not, it would be difficult to trace any constant 
connection between them. Even in £. scirpoides, in which the leaves 
are regularly three in a whorl, Schwendener has shown that these are not 
directly related to definite segments (Fig. g1).! The slender apices of 
Salvinia and Azolla have been held to show a constant relation of 
appendages to segments ; but even here, though a numerical correspondence 
may be traced, the successive leaves arise in different parts of the corre- 
sponding segments, being placed alternately in their upper and lower 
halves. In the Ferns there is not any regular numerical relation between 
1Schwendener, S#éz. ad. Ahad. 2 Berlin, 1885, pp. 927-933, Figs. 7, 8. 
