158 EDWARD C. JEFFREY OK 



Van Tieghem's ideas, more or less modified by 8trasl)urger's criticisms described 

 above, have met with general accei^tance and they are now t'ouuil in almost all botanical 

 text-books. As a consequence the morphological conceptions of Sachs and l)e Bary have 

 been almost universally abandoned. 



It is necessary to examine, whether these new ideas are really more in accordance 

 with the facts, than those of Sachs and De Bary, and that this may be done more definitely 

 the following citation from Van Tieghem's (Traite de botanique, 1891, p. 765) writings is 

 put before the reader. " Le cylindre central etroit et sans moelle de la region inferieure 

 de la tige, au lieu de se dilater, comme dans le cas ordinah-e, au lieu de se rompre en 

 faisceaux liberoligneux distincts, comme dans la structure astelique, s'elargit quelquefois 

 eu un rul)an. ([ui bientot se divise en deux par un etranglement median. Chatiue moitie 

 s'aplatit plus haut Ti son tour et se divise en deux et ainsi de suite." This is his descrip- 

 tion of the nuxle of development of his polystelic type. ' 



In his essay on polystely (Van Tieghem, Ann. sci. nat., Bot., ser. 7, torn. 3) we do 

 not find any specific instances, of the development of cryptogams, illustrative of the 

 polystelic type. In a later publication, however (Traite de botanique, 181)1, p. l.i)72), 

 he gives the following brief account of the development of the stem of Pteris 

 aquUina. ''La Pteride aquiline, par exiunple n'a d'abord. j usque vers sa septieme feuille, 

 q'une stele axile, plus haut, cette stele se divise en uue stele dorsale et une stele 

 ventrale, formant un cercle unique ; c'est plus tard seulement que ces steles produisent 

 des branches qui sejournent dans I'ecorce et y constituent un second cercle en dehors du 

 premier." As this is the only example so far as I am aware, of a desci'iptioii on his part 

 of the development of a cryptogamous stem of his polystelic type and as Pteris aquil'uia 

 presents the greatest complexity of bundle-arrangement found among our common ferns^ 

 the writer proposes to describe briefly his own investigation of this form. The account 

 here given does not depend on the study of isolated sections, but on that of numei'ous 

 complete series of sections of the young stem, from the region of the foot to that where 

 the characteristic arrangement of the bundles of the adult is reached. 



Above the exit of the first leaf-trace the young vascular axis is a bundle-tube, which 

 at first is so narrow, that no fundamental tissue is enclosed by it. Consequently in passing 

 inwards we meet, first the external phloem, then the ring of vessels and innermost a core 

 of phloem. At the height of origin of the fourth or fifth leaf, a core of fundamental tissue 

 makes its appearance inside the internal phloem. Immediately above the point of origin 

 of each leaf-trace there occurs a break in the continuity of the fibrovascular tube, which 

 may be called the foliar lacuna. In the younger region of the stem, the internal and 

 external bast-elements communicate through these lacunae, but, when the stem has 

 reached the stage described above, where there is present internally a core of fundamental 



