160 EDWARD C. JEFFREY ON 



of 11r' repeated dichotomy of the primitive strand wliich has l)een indicated by Van 

 Tieghem. The writer's results in the example described moreover agree with those of 

 Leclerc du Sablon (Ann. sci. nat. hot., ser. 7, tom. 11) for his figures and descriptions of 

 the youiKjer stages of the stem of Pti'i-'is aqin/iufi are ((uite in harmony with the state- 

 ments above. He has made the curious mistake, however, of regarding the mature 

 rhizome of this species, as derived from an adventitious bud on the young plant, although 

 Hofmeister's (Higher cryptogams, Ray soc, p. 213) account is correct in this respect. 



The writer's study of the development of the stem in a wide range of North Ameri- 

 can and exotic Pteridophyta has led to the result that, in the vascular axes of stems, 

 there are just two primitive types of stelar structure, viz., the single concentric strand, 

 and the tuljular concentric strand, and moreover that all the varied complexities of the 

 mature stem may readily be derived from these by the study of development in the 

 individual cases. 



Tliere can be little doubt that the simple concentric strand is the more primitive 

 type, and that the tubular concentric strand was subsequently derived from it. The 

 writer is inclined to attribute the origin of the tubular type to the action of mechanical 

 causes, but it would be out of place to discuss this subject at the present time. The two 

 primitive types of stele described above may, however, be appropriately designated, pi'O- 

 tostelic and siphonostelic respectively. 



In the protostelic axes of the Hymenoiihyllaceae we generally find the concentric 

 type of stele, but in the genus Hemiphlebium (Prantl, Die Hymenophyllaceen, Plate 

 4, figs. Gl, 02) the bast disappears on the lower side of the stele, which thus becomes 

 collateral. The Ophioglossaceae afford examples of this unilateral degeneration of the 

 bast in siphonostelic stems. From a study of the development of the young stem of 

 species of Ophioglossum and Botrychium the writer is able to state that there occurs 

 here the same stelar tube as in the young stem of Pferis aquU'ma. The two types differ 

 only in the absence of internal bast in the former. The researches of Poirault 

 (Recherches anat. sur les crypt, vasculaires, Ann. sci. nat., hot., ser., 7, tom. 8), and 

 Van Tieghem (Journ. de hot., 1890) have demonstrated the presence of an internal 

 endodermis in the young stem of various Ophioglossaceae. It is interesting also to note 

 tliat the leaf-traces of Bofrt/chium virginianium are concentric, and like those of the 

 cycads retain in all probability a primitive type of organization, which has disappeared 

 in the bundles of the stem. 



Our examples have up to the present been drawn from the Filicales. The primary 

 vascular axes of livhig Lycopodiales rarely present the phenomena of siphonostely. A 

 o-ood example, however, is to be found in Selaginella laevifjatu Blk., var. lynUii Spr. The 

 stelar tube in this case resembles that of Fteris aquilina and has likewise a pair of medul- 



