﻿STARR— AYTONIA 



Cells secreting mucilage and resinous oils appear singly in thal- 

 lus, receptacles, and scales; sometimes there is a layer of enlarged 

 mucilage cells on the lower surface. Mucilage hairs are common 

 about the receptacles and in the antheridial cavities. The middle 

 lamella of the radial walls of the epidermal cells shows a tendency 

 to become mucilaginous. Pitted cells occur in both the compact 

 and loose tissue; they are not elongated more than other cells 

 (%. 8). 



Development 



Growth takes place by divisions of a wedge-shaped apical cell 

 (fig. 9), as described by Leitgeb. From the lower segments scales 

 are formed almost immediately, but rhizoids not until later. The 

 epidermal cell giving rise to a rhizoid is often larger than the result- 

 ing rhizoid, forming a bulbous base. Division in the lower part 

 of the thallus is more rapid than in the upper, indicated by the 

 pulling backward of the bottoms of the air cavities. Leitgeb 's 

 theory of the origin of air cavities, as due to depressions in the sur- 

 face and the upgrowth of adjacent parts, was combated by Barnes 

 and Land (i) and by Petsch (16), but was upheld by Miss Black 

 (2) in regard to Riccia Frostii. My material leaves no doubt as 

 to their schizogenous origin (fig. 10). Leitgeb thought the spac es 

 in Plagiochasma became partitioned by the outgrowth of cell 

 plates. Campbell (4) also says that the original air chambers 

 become divided by the development of partial diaphragms into 

 secondary chambers. The cavities at first are always deep and 

 narrow, as noted by Barnes and Land, but they soon become wide, 

 irregular chambers by the stretching and tearing of tissues between 

 neighboring chambers, as shown by the irregularity of the resulting 

 surfaces and by torn cells, the tearing being due to the differences 

 in tension between the upper and lower parts of the thallus. This 

 leaves projecting plates of cells, appearing as filaments in section, 

 which Leitgeb and Campbell interpreted as new growth dividing 

 the original chambers. Perhaps these plates add to their length 

 by further growth. In fig. 11 projecting filaments a and b are 

 attached in the next section. 



The receptacles are formed in a direct line behind the growing 

 point. Leitgeb noted that they are not complete branches of 



