250 Evans: Air chambers of Grimaldia fragrans 



resemblance to that of Grimaldia. Its development has been 

 described by Miss Starr (lo), her investigation having been based 

 on an undetermined species from Mexico. She confirms the 

 earlier observation of Barnes and Land that the air chambers of 

 Plagiochasma owe their origin to a splitting of cell walls below the 

 surface. She notes further that the chambers are at first deep 

 and narrow but that they soon become wide and irregular, and 

 she ascribes the changes in size and form which they show to a 

 "stretching and tearing of tissues between neighboring chambers." 

 In other words she considers that schizogenous processes play a 

 leading part in the enlargement of the chambers as well as in their 

 origin. This conclusion is hardly supported by her /. ii or by 

 the earlier figures published by Barnes and Land (i, /. iy-22). 

 Although these figures indicate a schizogenous origin of the cham- 

 bers, they do not disprove that the enlargement is mainly due to 

 the growth of the surrounding cells. 



Summary 



The air chambers of Grimaldia fragrans are in several layers in 

 the thickened median portion of the thallus. 



The dorsal chambers communicate with the outside by means 

 of epidermal pores. They are subdivided by an irregular system 

 of more or less vertical, united cell plates, enclosing narrow spaces, 

 so that the boundaries of the chambers are difficult to distinguish. 

 The cell plates sometimes reach the epidermis and sometimes do 

 not; in the latter case the free margins sometimes bear scattered 

 teeth, less than two cells in length, especially in the vicinity of the 

 pores. Except for these teeth the chambers lack filaments com- 

 pletely. 



The more deeply situated chambers communicate with one 

 another and with the dorsal chambers by means of passageways ; 

 they are scarcely or not at all subdivided by cell plates. 



The chambers all owe their origin to a splitting of cell walls in 

 closely united tissue. In the case of the dorsal chambers the split 

 sometimes begins below the surface and extends outward; some- 

 times at the surface and extends inward. 



The dorsal chambers appear first, very close to the apical cell, 

 but the more deeply situated chambers appear soon afterwards. 



