424 Systematic Paleontology 



are apparently cut off from the cell cavity, and the third spine figured 

 gives a good idea of the papillose character of those adjacent cells in 

 which these processes are not prominently developed. 



In the area included in fig. 14 are three of the curious stomatal open- 

 ings which apparently characterize the genus Frenelopsis. These are 

 circular in outline and about 0.03 nun. in diameter. They are very 

 numerous, but whether they are localized on certain portions of the 

 branches which perform the functions of leaves in this genus, or whether 

 they are uniformly distributed on the annual shoots, could not be deter- 

 mined. They consist of five or six guard cells arranged around the 

 central stomatal opening. These cells are much thinner-walled than 

 the epidermal cells. In form they are relatively slender distad and 

 broad proximad. As viewed through the microscope, they are darker 

 colored around the stomatal opening and peripherally they are lighter. 

 Since structural material is not available, their exact attitude is made 

 out with difficulty. Their outer centrally directed ends come into focus 

 at about the same time as do the outer ends of the longer spinelike proc- 

 esses, or very soon after, while their inner broad ends are visible 

 after the epidermal cells have gone out of focus; hence it is obvious 

 that they are inclined toward each other and project outward for a con- 

 siderable distance from beneath the surrounding epidermal cells. 



In their more essential characters they agree with the stomata as 

 described by Zeiller for F. Hoheneggeri and by Velenovsky for F. 

 hohemica. Just what were the physiological factors responsible for the 

 great reduction of the leaves and the assumption of the photosynthetic 

 processes by the branches in Frenelopsis it is difficult to imagine. Such 

 features are usually associated with peculiarities of climate and habitat, 

 and suggest strong insolation and lack of humidity; but such conditions 

 are not suggested by the other members of the flora associated with 

 Frenelopsis, since with the Potomac species are found large numbers of 

 ferns, many of them apparently tree ferns with decompound fronds a 

 meter or more across, and large numbers of cycads of various genera 

 and large size; while in the latest beds in which F. ramosissima occurs 

 there are considerable numbers of dicotyledonous leaves, some of which 



