THE OCCURRENCE OF “ROOT GAPS” IN THE RHIZOME SOLENOSTELE AND LEAF TRACE OF DENNSTAEDTIA CICUTARIA

The vascular system of the rhizome axes of Dennstaedtia cicutaria consists of a solenostele with an amphicribral vascular bundle centrally located in the pith. Each leaf has a single trace which is an amphicribral vascular bundle. At each node of an axis there is a complex consisting of the main axis, leaf base, and a branch axis attached to the basiscopic margin of the leaf base. Numerous roots are present on the rhizomes and the abaxial side of the leaf bases. Parenchymatous gaps occur in the rhizome solenostele and the leaf trace directly above the departure of some of the root traces. These gaps are termed root gaps. In some instances the root gaps are confluent. However, not all of the root traces have an associated root gap. The leaf trace is inserted laterally on the main and branch axes at the node so that the acroscopic leaf trace margin anastomoses with the main axis of the vascular system and the basiscopic margin with that of the branch axis. Two leaf gaps are associated with each leaf trace, one occurring in the main axis solenostele and the other in the branch axis solenostele. The medullary bundle of each axis anastomoses with each leaf trace at its point of attachment to the rhizome solenostele. Thus, the medullary bundle forms a continuous vascular strand from leaf trace to leaf trace in any given rhizome axis. METTENIUS (1857) observed that there were parenchymatous gaps unrelated to either leaf or branch traces in the solenosteles2 of Saccoloma elegans Kaulf., Stenochlaena tenufolia (Desv.) Moore, and many other rhizomatous ferns. This same phenomenon was also recorded by Kuhn (1889) in the solenostele of Christensenia aesulifolia (Blume) Maxon3 (as Kaulfussia aesulifolia Blume), by Brebner (1902) in the dictyostele of Danaea simplicifolia Rudge, by Ford (1902) in the dictyostele of Ceratopteris thalictroides (L.) Brongn. [(C. siliquosa (L.) Copel.], by Tansley and Lulham (1904) in the stele of Pteridium aquilinum (L.) Kuhn (as Pteris aquilina L.), and by Ogura (1938) in the dictyostele of Polypodium lasiostipes Mett. Gwynne-Vaughan (1903) in his extensive investigation of the vascular system of several solenostelic fern taxa noticed that the same type of unrelated gaps occurred in both the solenostele and leaf trace of Dennstaedtia rubiginosa (Kaulf.) 1 Received for publication 11 June 1973. The author expresses his sincere gratitude to Ernest M. Gifford, Jr. for his invaluable advice and encouragement throughout the course of this investigation and for his critical reading of the manuscript. 2 The term solenostele is being used in the strict sense intended by Gwynne-Vaughan (1901) to denote an amphiphloic siphonostele without overlapping leaf gaps, whereas dictyostele refers to an amphiphloic siphonostele with overlapping leaf gaps, as proposed by Brebner (1902). 'The taxonomic nomenclature used in this report follows that used by Copeland (1947) except for the genus Dennstaedtia for which that of Tryon (1960) is used. Moore (as Dicksonia rubiginosa Kaulf.) but not in the vascular system of Dennstaedtia cicutaria (Sw.) Moore (as Dicksonia cicutaria Sw.). However, he appears to have investigated erroneously identified taxa, because Tryon (1960) does not maintain Dennstaedtia rubiginosa as a distinct species but validly recognizes it as D. cicutaria. Since Gwynne-Vaughan did not file voucher specimens, it is virtually impossible to ascertain with any accuracy which genus or species he investigated; nevertheless, his observations remain quite pertinent. He termed the gaps which were unrelated to appendage traces "lacunae" and considered a solenostele with lacunae as being a peculiar type of dictyostele which had evolved without overlapping leaf gaps. Thus, he concluded that this type of stele could still be considered a true solenostele. On the other hand, Tansley (1907a) applied the term "perforation" to this type of gap and designated all steles having perforations as "perforated steles." Thus, Tansley (1907b, 1908b) used such terms as perforated solenostele and perforated dictyostele. He considered these perforations to be the result of a reduction of the stelar system and concluded that vascular tissue had failed to differentiate in the perforations (obviously an oversimplified conclusion that cannot be disputed even when applied to leaf and branch gaps). He (1908a) also inferred that a perforated stele was formed as the result of an increase in the transectional stelar diameter and, furthermore, that it was a phenomenon similar to