THE THALLOSE HEPATICAE OF THE JUAN FERNANDEZ ISLANDS 



581 



dichotomy but often do not branch at all b\' this method. In normal cases 

 the growth is limited, although no examples of true apical wings have been 

 observed. At the same time a reversionary process is not infrequent, whereby 

 a thallus becomes narrower or may even continue its growth as a wingless 

 rhizome. 



In addition to the forking of the winged thallus, intercalary branching may 

 occur. The intercalary branches arise very sparing!}- either from the rhizome 

 or from the ventral surface of the winged thallus in the lower part. They easily 

 become detached, thus forming new independent plants and representing a tjpe 



Fig. 6. Pallavicinia xiphoides (Tayl.) Steph. 



A. Transverse section of a midrib, showing strand of tracheidal tissue and parts below, x 225. 



B. Transition-region between midrib and unistratose portion of wing, X 225. C, D. Outer 

 portions of the same wing in transverse sections, X 225. E — I. Marginal teeth, X 100. A — G 



were drawn from no. 91 and the remaining figures from a New Zealand specimen. 



of vegetative reproduction. In this wav' extensive mats of plants may be 

 produced. 



The strand of tracheidal tissue in the rhizome extends into the midrib of 

 the winged thallus, where it sometimes increases to 40 a in width. In the case of 

 a dichotomy the forking of the thallus is preceded by a forking of the strand; 

 but, in the case of an intercalary branch, the strand arises de novo and is not 

 continuous with the strand of the original thallus. This condition, which of 

 course facilitates the separation of the branch, was first noted by Leitgeb^ 

 in the case of P. Lyellii (Hook.) S. F. Gray and is clearly shown in Campbell 



' Untersuch. iiber Leberm. 5 : 80 (1877). 



