246 HEREDITY AND EVOLUTION IN PLANTS 



the Cycadean series (Cycadales, Hemicy cad ales (Bennet- 

 titales) , Cycadofilicales) which have pithy stems; but some 

 of the Cordaitean forms also have pithy stems and com- 

 paratively large leaves. Here again, as so often, an 

 attempt at a formal classification necessitates drawing 

 an apparently sharp line where in fact one does not exist. 

 As Professor Jeffrey 1 has said, the term Archigymno- 

 spermac is one of convenience, and like most scientific 

 terms falls short of covering the situation. On the basis 

 of certain criteria (e.g., the structure of the wood), the 

 Ginkgoales appear to be intermediate between the Coni- 

 ferales and the Cordaitales. In fact, as Jeffrey 2 has pointed 

 out, the "living fossil," Ginkgo, may be regarded as a 

 connecting link or transitional form between the Archi- 

 gymnospermae and the Metagymnospermae. 



The relationship of Isoetes is one of the most difficult 

 to determine among all the vascular cryptogams. Argu- 

 ments for and against interpreting it as derived by re- 

 duction from the Lepidodendron group are given by Lady 

 Isabel Browne. 3 The secondary growth in thickness of 

 its stem (in such a dwarfed form) must be regarded as a 

 character of long standing, not recently acquired; plants 

 in both groups have mucilage cavities. Isoetes resembles 

 some of the Lepidodendrales (e.g., the so-called Stigmaria*} 

 in the dichotomous branching of its roots. Other facts 

 of structure (e.g., the occurrence of the sporangia on the 

 upper side of the leaves) have also been interpreted 



'Jeffrey, E. C Science, N. S. 47: 316. 1918. 



2 Jeffrey, E. C. The anatomy of woody ptanls, p. 315. Chicago, 1917. 



3 Browne, Lady Isabel. The phylogeny and inter-relationships of the 

 Pteridophyta. New Phy tola gist 7: 93, 103, 150, 181, 230. 1908. 



* The fossil remains to which the generic name Stigmaria was assigned 

 have long been known to be the root-system of Sigillaria. 



