I ;6 



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 



FIG. 91. 



A Apex of Equisetum 'scirpoides : the swelling below shows the highest leaf sheath : this 

 extends upwards on the right to the segment-wall, on the left only to the middle of a segment. 



= trans, sec. of t)he 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, //^principal walls. 6'^sextant walls; the position of the leaves is 

 independent of these. X 550. (After Schwendener.) 



segmentation at the apex. The genus Equisetum 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 E. scirpoides, in which the leaves 

 are regularly three in a whorl, Schwendener has shown that these are not 

 directly related to definite segments (Fig. Qi). 1 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 

 1 Schwendener, Sitz. d. Akad. zn Berlin, 1885, pp. 927-933, Figs. 7, 8. 



