METHODS 



II 



The cells of the ventral part of the midrib have thick walls in which 

 are pits elongated transversely. These pits are in lines extending round 

 the cell, and forming partial spirals, this being perhaps due to the longi- 

 tudinal growth of the cell (Fig. 3, D, E). These cells, which are pre- 

 sumably conducting cells, have very little protoplasmic contents, thus 

 agreeing with Tansley's description of the corresponding tissue in other 

 members of the genus [1]. The cells of the cortical tissue of the rhizome 

 contain many starch granules, in this respect recalling Cavers' figures of 

 Morkia flowtowimia [2]. 



A. 



Fig. 5. Pallavicinia Zollingeri. 

 Two fronds of female plants, x 2. 



The base of the aerial shoot, or frond, like the rhizome of which it is 

 the continuation, is quite destitute of the wing-like lamina found in the 

 expanded portion of the frond. The cylindrical stalk, as it ascends, be- 

 comes gradually flattened dorsi-ventrally and develops a narrow wing 

 on either side which widens until it forms the beginning of the expanded, 

 fan-shaped lamina of the frond. The first dichotomy occurs when the 

 young frond is 3-4 cm. in height. The forking is repeated from two to 

 four times, and there thus results the palmately divided leaf-like shoot, the 

 slender central strands of the delicate segments simulating almost exactly 

 the venation of a true leaf. In the archegonial plants these fan-shaped 



