STUDIES AMONG OUR COMMON HEPATIC/^. 



By Alexander W. Evans. 



IV. PORELLA PLATYPHYI.LA (L.) Lindb. 



THE subject of our present study is probably the commonest of 

 all the leafy hepatics in eastern North America, and it is so 

 large and striking in its appearance that it can hardly be over- 

 looked even by the beginner. As in all our previous types, its 

 range extends across Europe and northern Asia ; it is also found in our 

 own western country, but is rare there and is largely replaced by other 

 members of the genus.* In the lowlands it is perhaps commonest on 

 shaded rocks, while in the mountains it is frequent on the trunks of 

 trees; it is, however, by no means confined to such localities, and may 

 be found also on the earth or on fallen logs. 



The young plants are prostrate and adhere pretty closely to the 

 substratum by means of clusters of rhizoids springing from their 

 underleaves. Very soon the creeping, primary stem and its prostrate 

 branches give rise to secondary stems, which cover over the prostrate 

 part, shut out its light and gradually cause its death and partial decay. 

 In plants growing on a level or slightly inclined surface, these sec- 

 ondary stems are also procumbent, but develop very few rhizoids, and 

 consequently do not adhere closely to the substratum ; on tree-trunks 

 or steep rocks, on the other hand, the stems first droop and then 

 curve outward so as to bring their free ends into an almost horizontal 

 position. It is these procumbent or drooping secondary stems which 

 we commonly meet with and to which we will devote our attention. 

 Very often they form pure mats of the Porella extending over consid- 

 erable surface and completely hide from view the decayed and almost 

 indistinguishable primary stems. 



The secondary stems (Fig. i) are very robust; they are 3-8*='^ 

 long and, with their leaves, measure 2-3 m'^- in width. In cell-struc- 

 ture they show a slight advance upon the other three plants which we 

 have studied. Their cells are still closely packed together without 

 intercellular spaces, and those of the interior are very similar to those 

 which we found in Jungcrmannia barbata, but, on the outside of the 

 stem and forming a more or less distinct cortex three or four cells 

 thick, we find slightly smaller cells with very much thickened walls. 

 These cortical cells not only strengthen the stem, but are apparently 



*ln ihe Bulletin of the Torrey Botanical Club for November, 1897, Dr. Marshall A. Howe has 

 published an account of "The North American Species of Porella." This article has been of 

 much assistance to the writer in the preparation of the present paper. 



