Igor | ANATOMY OF THE OSMUNDACEAE 407 
endodermis may fuse between the bundles, but in no case is there 
an inward looping of the endodermis. Finally, the stelar origin of 
the pith of the medullated monostele has been disputed, and the 
question raised as to whether the medullary and cortical tissues 
are in reality morphologically different. In other words, is the 
medullated monostelic type primitive, as its simplicity might 
indicate, or has it resulted by degeneration from more complex 
types ? 
It is interesting to note that Potonié had discussed this last 
question from the standpoint of fossil botany, and concludes™ 
that it seems evident in the case of certain groups, such as the 
cycads, that the simple results from the complex (for example, 
the cycads from the Medulloseae). Hence for these groups at 
least he is inclined to reject this idea of segregation of parenchyma 
in the center of the protostele to form the medullated monostele, 
but holds that the medullated monostelic type has probably 
arisen by degeneration from his “pericaulom.” Since this peri- 
caulom was produced, according to his theory, by the lateral 
fusion of leaf bases in the stem surrounding the originally solid 
stele, the ‘‘urcaulom,” the medullated monostele has been derived 
from a form of central cylinder such as Van Tieghem has 
described as polystelic, preceded or accompanied by the dis- 
appearance of the enclosed urcaulom. The paleontological 
evidence, however, appears not to be conclusive, for in the very 
group that Potonié cites, the cycads, so eminent a paleobotanist 
as Dr. D. H. Scott takes a directly opposite view. He points 
out that the vascular system of the Medulloseae was typically 
polystelic, while in the recent cycads there is but one vascular 
cylinder, and that hence “we should involve ourselves in unneces- 
sary complications if we endeavored to derive the simple, primary 
Structure of the cycadean stem from the more elaborate organi- 
zation of a Medullosa. It is far more natural to suppose that 
the monostelic cycads arose from monostelic ancestors.” 
PoToniE: Die Metamorphose der Pflanzen im Lichte palzontologischer 
Thatsachen 22. 
33Scotr: Studies in fossil botany 395. 1900. 
