PAST HISTORIES OF PLANT FAMILIES 155 



treelike form of Sphenophyllum has been discovered, 

 and in the structure-petrifactions the largest stems we 

 know were less than an inch in diameter. 



In the internal anatomy of these stems lies one of 

 the chief interests and peculiarities of the plants. In 

 the very young stage there was a sharply pointed solid 

 triangle of wood in the centre (fig. 113), at each of the 

 corners of which was a group of small cells, the proto- 

 xylems. _ The structure of such a stem is like that of 

 a root, in which the 

 primary wood all 

 grows inwards from 

 the protoxylems to- 

 wards the centre, and 

 had we had nothing 

 but these isolated 

 young stems it would 

 have been impossible 

 to recognize their true 

 nature. 



J } to Fig. 113. — Sphenophyllum, Transverse Section of 



stems are rare, for the Young stem 



development 01 sec- c, cortex, the soft tissue within which has decayed 



OndarV wood be^an ^""^ ^^^^ ^ space, in which hes the soh'd triangle of 



1 J . *' wood, with the small protoxylem groups fx at each 



early, and it soon corner. (Microphoto.) 



greatly exceeded the 



primary wood in amount. Fig. 1 14 shows a photograph 

 of a stem in which the secondary wood is well developed. 

 The primary triangle of wood is still to be seen in the 

 centre, and corresponds to that in fig. 113, while closely 

 fitting to it are the bays of the first-formed secondary 

 wood, which makes the wood mass roughly circular. 

 Outside this the secondary wood forms a regular cylinder 

 round the axis, which shows no sign of annual rings. 

 The cells of the wood are large and approximately square 

 in shape, while at the angles formed at the junction of 

 every four cells is a group of small, thin-walled paren- 

 chyma, see fig. 1 1 5. There are no medullary rays going 



