216 



BOTANY 



bers of the latter group conform invariably to a single structural 

 type. The apex of the shoot (Fig. 176) is occupied by a single 

 apical cell which, except in the genus Physiotium, is tetrahedral 

 in form. In transverse section it appears as a triangle, which is 

 usually isosceles, with the shorter side turned toward the ventral 

 surface of the shoot. There are three series of segments formed, 

 corresponding to the three lateral faces of the apical cell, and each 



B. 



FIG. 176. Madotheca Bolanderi. A, longitudinal, B, transverse, section of a vege- 

 tative shoot (X 500) ; d, dorsal, v, ventral, surface. C, longitudinal section of 

 antheridial, Z>, of archegonial, shoot; 5, mother-cell of antheridium ; ?, young 

 archegonium. 



segment gives rise to a leaf, except where the ventral face of the 

 apical cell is very narrow, in which case the ventral series of leaves, 

 the " Amphigastria," are not developed. 



The fully developed shoot shows a definite central axis, upon 

 which the leaves are arranged in three rows, two dorsal and one ven- 

 tral. The dorsal leaves are usually two-lobed, the lobes being either 

 of equal size, or, more commonly, the upper lobe is larger, and over- 

 laps the lower lobe of the leaf in front of it. The leaves and stem 



