i 4 2 SCIENCE PROGRESS. 



junctive system, is much needed to complete our know- 

 ledge of these matters. 



An important modification of the theory of steles has 

 been made by Van Tieghem himself in extending the use 

 of the term astely so as to make it include the state of 

 things obtaining in the stems of all species of Equisetum 

 (12), and of p hio gloss ace ce (13). 



Let us take first the case of Equisetum. Well- 

 marked endodermes are found in the stems of all species, 

 but their disposition, which was fully worked out many 

 years ago by Pfitzer, is very various, not only in different 

 species, but in different parts of the stem of the same 

 species. There are three types of arrangement. In the 

 first each vascular bundle is surrounded by a special endo- 

 dermis ; in the second the ring of bundles is bordered within 

 and without by a general endodermis ; and in the third 

 the outer endodermis alone is present. In the second 

 edition of the Traite Van Tieghem assigned the first 

 two conditions to the astelic, the third to the monostelic 

 type, but in a paper (12) published in the same year (1890) 

 he calls attention to the fact, discovered by Pfitzer, that 

 all the species possess, at their nodes, the first or second of 

 the arrangements in question. He therefore concludes 

 that all belong really to the astelic type, and that where, 

 for instance, the second type, just above a node, passes 

 back into the third, we have simply a case of the dis- 

 appearance of the special characters of the inner endo- 

 dermis, which must still be supposed to exist. The " mono- 

 stely" is only apparent, and the tissue bordering the 

 central canal of the stem, internal to the inner (theo- 

 retical) "endodermis," is not in reality pith, but rather 

 " inner cortex" (extra-stelar tissue). The first of the three 

 arrangements is to be called dialydesmic, since each bundle 

 with its sheath of conjunctive is separate ; the second and 

 third gamodesmic, since the conjunctive tissue surrounding 

 the bundles is in lateral confluence. 



Turning now to the Ophioglossacece we have a similar 

 argument (13). The stem of Ophioglossum vulgahtm, 

 below the level of the first leaf, is monostelic, but above the 



