254 PTERIDOPHYTA. [CH. IX 



protoxylem in the xylem of the root stele. The axial portion is 

 occupied by large tracheids, and the smaller tracheids of the 

 xylem occur as radially disposed groups, alternating with groups 

 of phloem. External to the xylem and phloem strands there 

 occur two layers of cells, usually spoken of as a double endo- 

 dermis, but it has been suggested that it is preferable to 

 describe the double layer as the phloeoterma^, of which the 

 inner layer has the functions of a pericycle, and the outer that 

 of an endodermis. A transverse section of a root is seen in 

 fig. 55, 1, the dark cells on the left are part of a thick band of 

 sclerenchyma in the cortex of the root, the layer e is the outer 

 layer of the phloeoterma. 



Without describing in detail the development^ of the 

 sporangia, it should be noted that the sporangial wall is at first 

 3 to 4 cells thick, but it eventually consists of a single layer. The 

 cells have spiral thickening bands on the ventral surface, and 

 rings on the cells where the longitudinal splitting takes place. 

 Each sporangium is supplied by a vascular bundle which is 

 given off from that of the sporangiophore axis. The strobili 

 are isosporous. 



, I. EQUISETITES. 



I II. PHYLLOTHECA. 



FOSSIL EQUISETALES. jni. SGHIZONEURA. 



IV. CALAMITES. 



' V. ARCHAEOCALAMITES. 



In dealing with the fossil Equisetales, we will first consider 

 the genera Equisetites, Phyllotheca and Schizoneura, and after- 

 wards describe the older and better known genera Calamites 

 and Archaeocalamites. A thoroughly satisfactory classification 

 of the members of the Equisetales is practically impossible 

 without more data than we at present possess. It has been the 

 custom to include Equisetites, Phyllotheca and Schizoneura in 

 the family Equisetaceae, and to refer Calamites and Archaeo- 



1 Strasburger (91) p. 435. 



2 Bower (94) p. 495. 



