Notes. 
5i5 
tissue. This central cylinder is common to stem and root, though 
the two organs differ in the distribution of its constituent tissues. 
In spite of the almost infinite anatomical diversity which is met 
with among Phanerogams, this monostelic structure is scarcely ever 
departed from. Whatever may be the arrangement of the wood and 
bast, however much the differentiation of the conjunctive tissue may 
vary, whether there be indefinite cambial increase or not, or whatever 
form this increase may take, still the presence of a single central 
cylinder is an almost constant character. 
In the Vascular Cryptogams this is not the case. Although in 
many Equisetums, in Lycopodium , Isoetes, some species of Selaginella , 
and some of the simpler Ferns, the axis is monostelic, in the great 
majority of the Ferns, and in many Selaginellas a different type 
prevails. In these the central cylinder no longer remains simple ; it 
bifurcates repeatedly, and the mature stem is traversed by a number 
of distinct, but anastomosing, vascular cylinders (the “ concentric 
bundles ” of De Bary’s Anatomy), each of which is homologous with 
the single cylinder of a monostelic stem. It is, however, a fact of 
essential importance that in all vascular plants whatsoever the embryonic 
structure is monostelic, the stem, like the root, primarily containing a 
single cylinder of small diameter and simple organization. In the 
flowering plants progress takes the direction of increase in the size 
and complexity of the one cylinder ; in the Cryptogams above cited 
a complex conducting system is attained in another way ; the original 
cylinder branches and the stem becomes polystelic , in all the later- 
developed internodes 1 . 
Van Tieghem, the originator of the anatomical conceptions which 
have just been indicated, well says that polystely is the most impor- 
tant modification which the structure of the stem can undergo 2 . 
According to our present knowledge there are only two genera of 
flowering plants in which polystely occurs, namely Auricula 3 and 
Gunnera. Though systematically so remote from each other, these 
genera agree, so far as most of their species are concerned, in posses- 
1 Leclerc du Sablon , Recherches anatomiqnes sur la formation de la tige des 
Fougeres. Annales des Sci. Nat, Bot., Ser. VII, t. xi, 1890. 
2 Traite de Botanique, 2 me . ed. 1891. Pt. 1, p. 767. See also Van Tieghem et 
Douliot , Sur la polystelie. Ann. des Sci. Nat, Bot, Ser. VII, t. iii, 1886. 
3 Auricula is usually regarded as a section of Primula. Van Tieghem makes 
it a separate genus, and his nomenclature is provisionally adopted here, though I 
express no opinion on the systematic question. 
