rvr.\i>(>i"M,i('\i,KS 9 



ptcridophylcs. as for cxamplo in Osniundaci-ai' among ferns. In 

 tlu' usual sc(|U(,'ni I' of forn families, \\\v polystelic condition is found 

 at l)olh extremes; and the confessedly primitive ])rotostelic condition 

 j)crsists in the midst of the fern series. The inference is that in \arious 

 ways, along \arious lines, ihanges in the i)rimili\c' protostelic con- 

 dition ha\'e develo|)i'd. To arrange all the tyj)es of stele in a linear, 

 ])hylogenetic series seems to be not only unnecessary, but also con- 

 trary to all previous exj)erience with phylogenies. 



Assuming that thi' polystele is nuTely a split up siphonostele 

 (31. .^0), there are just two types of ci'iitral cylindi'r, the protostele 

 and the siphonostele, the former being the mori' jjrimitive. Jkkfrky 

 has shown (31) that siplu)nosteles are of two fundamental tyjies: 

 (i) those with both leaf and brainh gajjs {phyllo.si phonic), and (2) 

 those with bramii gaps alone {cludosi phonic), 'i'he |)hyll()si|)honic 

 condition is associated w ilh large leaves, and the cladosiphonic with 

 small ones; and therefore Jkffrey recognizes two great primitive 

 stocks of \ascular plants: the Lycopsida, which are cladosiphonic 

 and microphyllous; and the Ptcropsida, which an.- phyllosi|)honic 

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

 sperms, and angiosperms. This association of fi-rns and gymno- 

 sperms is a strong cont'irmalion of the conclusion reached in other 

 ways as to their ])hylogenetic connection. 



The leaf gap, characteristic of the I'leropsida, should be under 

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

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

 facing each other (lig. ()), one being a smtion of the leaf trace, the 

 other a section of the siphonostele which the li'af gap has madi' 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 tlu' latter should bi' recog- 

 nized l)y the fact that it occurs inunediately above the leaf trace to 

 which it belongs. A true leaf ga]) is always related to a singli- trace, 

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

 cylinder, and esi)ecially if they are related to the sides of the ga|), it 

 is not a true leaf gaj). It should also be noted that ga[)s in the vascu- 

 lar cylinder may occur where the vascular tissues are much reduced 



