242 Evans: Air chambers of Grimaldia fragrans 



It is clear from a comparison of transverse, longitudinal and 

 horizontal sections that a distinction may be made between the 

 dorsal layer of air chambers in Grimaldia and the more deeply 

 situated layers. In the dorsal layer the original chambers show a 

 secondary partitioning by a system of more or less vertical cell 

 plates, the free margins of which sometimes bear scattered teeth, 

 apparently always less than two cells long. Except for these 

 teeth the chambers lack filaments completely. In the more 

 deeply situated layers, the chambers are much simpler and rarely 

 show evidences of any kind of outgrowth. These conclusions 

 show the incorrectness of certain statements made by Stephani, 

 K. M tiller, and Massalongo and the essential correctness of Schiff- 

 ner's account. 



The complex conditions found in the green tissue of Grimaldia 

 are duplicated by Plagiochasma and by certain species of As- 

 terella. The other genera showing the Reboiilia type of air cham- 

 ber have a much looser green tissue, the secondary partitioning 

 being less highly developed or absent altogether. 



Origin and enlargement of the air chambers 

 The development of the air chambers in the Marchantiales 

 has aroused a good deal of interest among students of the Hepati- 

 cae, and the history of the subject is fully given by Barnes and 

 Land (i). The explanation which they advance to account for 

 the origin of the chambers differs in certain respects from the older 

 explanation advanced by Leitgeb and accepted by many of his 

 successors. Leitgeb's explanation was based primarily on his 

 study of Riccia, but he extended its application to the more com- 

 plex genera. According to his ideas the air chambers do not 

 originate in compact tissue, and no splitting of cell walls is involved 

 in their formation. They arise, rather, on the surface of a young 

 thallus and are due to a cessation of upward growth in certain 

 limited areas, the surrounding parts growing upward vigorously. 

 The areas where growth is supposed to cease are situated in most 

 cases where four of the surface cells come together; they mark the 

 lower ends of the chambers, the vertical extent of which depends 

 upon the degree of upward growth which the surrounding parts 

 exhibit. 



