150 I'SAEONIUS. 



Professor Zeiller has given a very full description of the detailed 

 anatomy of this stem, from which the above account is in the main 

 derived. 



The essential features of the complex structure of the vascular 

 cylinder can be made out from the photograph of one of the British 

 Museum slabs reproduced on Plate VII. The central cylinder 

 is in form nearly square, with rounded angles. For the sake of 

 clearness we shall describe the steles with reference to the sides and 

 angles of this square. We may first consider the steles bordering 

 on the sides of the square. At the upper and lower borders there 

 are two well-marked invaginations of the sclerenchymatous band, 

 which is clearly seen completely surrounding the central cylinder. 

 The tissues in these two bays are not preserved. These invagi- 

 nations, however, mark the position at which two outgoing leaf- 

 traces have just become free from the cylinder, and have passed 

 out to supply a pair of opposite or sub-opposite leaves. At the 

 sides of the square two long and slightly curved steles are seen. 

 These will pass out from the cylinder as leaf-traces at a higher 

 level to supply the next pair of leaves alternating with, and 

 situated above the first pair. 



The third pair of leaf-traces will arise in the same position as the 

 first, i.e. at the upper and lower sides of the square. The steles 

 lying immediately below the invagination at the upper side of 

 the square will anastomose to form one of these leaf-traces. Thus 

 the four sides of the square are, as a rule, occupied by four leaf- 

 traces supplying four vertical rows of leaves. 



The diagonal corners of the square are occupied by four long, 

 curved steles, lying just inside the sclerenchymatous band, and 

 projecting slightly beyond the leaf-traces. These we may speak 

 of as the peripheral steles. It is from these peripheral steles that 

 the adventitious roots arise. They also contribute to the leaf- 

 traces, with which they anastomose before the latter become free 

 from the central cylinder. 



The internal portion of the square is occupied by several smaller 

 steles, which anastomose both with the more external steles, and 

 among themselves. 



Psaronius brasiliensis is one of the most beautiful of fossil plant- 

 remains, the colours, especially of the pseudo-cortex, as seen in 

 polished slabs, being exceptionally striking. 



