22 



BOTANICAL GAZETTE 



[JULY 



been entirely pushed away and the top of the spore mass presses 

 directly on the epidermis, the arch is ruptured at the center. 



Development of the peridium 



The peridial cells make their appearance with the enlargement 

 of the spore mass. The first peridial cells are seen at the apex of 

 the dome-shaped spore mass, and are first distinguishable from the 

 aecidiospores when the central spore chains are about 5 or 6 spores 

 in length (text fig. 8). These first-formed peridial cells are pro- 

 duced by a metamorphosis of the apical aecidiospores of the central 

 chains that involves an enlargement of the aecidiospore in all 



Fig. 8. — a, early stage in the formation of the peridium of P. Claytoniata; 4 

 peridial cells have been differentiated, all of which are apical cells of interior spore 

 chains; b, a more completely formed peridium; n peridial cells are shown, 7 of 

 which are from interior chains, and 4 (3 on the left and 1 on the right) are from the 

 peripheral chains. 



dimensions, a decrease in the density of its cvtonlasm 



thickening of its walls. 



le mature 

 mature a 



more 



The length of the cell is greater than the breadth, and its 

 long axis is parallel to the axis of growth of the spore chain in which 

 it is borne. This feature of the peridial cells is of some value in 

 determining their point of origin, as will be seen later. 



An early stage in the development of the peridium of P. Clay- 

 toniata is shown in text fig. 8, a. Only 4 peridial cells have been 

 differentiated in this section. The walls of these are drawn with a 

 double line. Three of them are apical cells of central spore chains, 

 and the fourth is the apical cell of the fourth chain from the right- 



