PAST HISTORIES OF PLANT FAMILIES 127 



than the axis itself. Together with the petiole usually 

 come off adventitious roots, as is seen in fig. 88, which 

 shows the main axis of a Botryopteris. The petioles of 

 the group show much 

 variety in their struc- 

 ture, and some are 

 extremely complex. 

 A few of the shapes 

 assumed by the steles 

 of the petioles are 

 seen in fig. 89; they 

 are not divided into 

 separate bundles in 

 any of the known 

 forms, as are many 

 of the petiole steles 

 of other families. 



In one genus of 

 the family secondary 

 wood has been ob- 

 served. This is highly suggestive of the condition of 

 the stele in Heterangium, where the large mass of the 

 primary wood is surrounded by a relatively small quan- 



Fig. 88. — Main Axis of Botryopteris witli simple 

 solid Protostele x. A petiole about to detach itself 

 f and the strand going out to an adventitious root 

 r are also seen, (Micro-photograph.) 



B 



D 



«a>o 



Fig. 89. — Diagrams showing the Shapes of the Steles in some of the Petioles of 

 different Genera of Botryopterideas 



A, Zygopteris; ^, Botryopteris; C, T-iibicaulis', '£), Aste7-ochlaena. 



tity of secondary thickening, developed in normal radial 

 rows from a cambium. 



Another noteworthy point in the wood of these plants 

 is the thickening of the walls of the wood cells. Many 

 of them have several rows of bordered pits, and are, 

 individually, practically indistinguishable from those of 



