CYCADOFILICALES 9 



pteridophytes, as for example in Osmundaceae among ferns. In 

 the usual sequence of fern families, the polystelic condition is found 

 at both extremes; and the confessedly primitive protostelic condition 

 persists in the midst of the fern series. The inference is that in various 

 ways, along various lines, changes in the primitive protostelic con- 

 dition have developed. To arrange all the types of stele in a linear, 

 phylogenetic, series seems to be not only unnecessary, but also con- 

 trary to all previous experience with phylogenies. 



Assuming that the polystele is merely a split-up siphonostele 

 (31, 36), there are just two types of central cylinder, the protostele 

 and the siphonostele, the former being the more primitive. Jeffrey 

 has shown (31) that siphonosteles are of two fundamental types: 

 (i) those with both leaf and branch gaps (phyllosi phonic) , and (2) 

 those with branch gaps alone (cladosiphonic) . The phyllosiphonic 

 condition is. associated with large leaves, and the cladosiphonic with 

 small ones; and therefore Jeffrey recognizes two great primitive 

 stocks of vascular plants: the Lycopsida, which are cladosiphonic 

 and microphyllous; and the Pteropsida, which are phyllosiphonic 

 and megaphyllous. To the latter stock belong the ferns, gymno- 

 sperms, and angiosperms. This association of ferns and gymno- 

 sperms is a strong confirmation of the conclusion reached in other 

 ways as to their phylogenetic connection. 



The leaf gap, characteristic of the Pteropsida, should be under- 

 stood clearly. A transverse section of the vascular cylinder of Adian- 

 tum, passing through a leaf gap, shows two crescentic vascular strands 

 facing each other (fig. 6), one being a section of the leaf trace, the 

 other a section of the siphonostele which the leaf gap has made incom- 

 plete. At the edge of the gap the external and internal phloems 

 become continuous. Farther up the stem the gap closes, and the 

 cylinder becomes complete again. There is sometimes danger of 

 mistaking a branch gap for a leaf gap, but the latter should be recog- 

 nized by the fact that it occurs immediately above the leaf trace to 

 which it belongs. A true leaf gap is always related to a single trace, 

 so that if several traces appear in relation to a gap in the vascular 

 cyhnder, and especially if they are related to the sides of the gap, it 

 is not a true leaf gap. It should also be noted that gaps in the vascu- 

 lar cylinder may occur where the vascular tissues are much reduced 



