THE GENUS EQUISETUM. 159 



tissue, the lattei- coramuuiciites pevioilicull)' also with the fundamental tissue outside the 

 tubulai" axis. The writer has not l)een able to discover here anj' indication of the repeated 

 dichotomous division which Van Tiegheni describes as characteristic of the stele of the 

 young polystelic axis. On the contrary, if his observations are correct, there is present 

 from the very first, a hollow tubular fibrovascular axis. 



After from nine to twelve leaves have been formed in this way, the young stem, 

 which up to the present has grown perpendiculai'ly upwards, bifurcates and the two equal 

 divisions plunge into the soil, and henceforth pursue the horizontal course, which is char- 

 acteristic of the adult rhizome. In consequence of this horizontal course, the leaves no 

 longer originate in a s}>iral manner, as in the younger upright axis, but come off alter- 

 nately from the sides of the young rhizome. As a result, the foliar lacunae occur on 

 opposite sides of the fibrovascular tube and frequently overlap, so that in certain planes of 

 cross-section, there is presented the appearance of independent dorsal and ventral steles. 



In the meantime a rod of bi'own sclerenchyma, oval in ti'ansverse section, has made 

 its appearance in the midst of the fundamental tissue occupying the center of the stelar 

 tulje. At a point about two or three centimeters from the region of bifurcation of the 

 young rhizome, the dorsal wall of the stelar tube becomes involuted, and gives off a bundle 

 into its cavity, which is quickly siu'rounded Ijy a tubular sclerenchymatous sheath, formed 

 by the bending round it from l)elow, of the rod of brown sclerenchymatous tissue already 

 mentioned. The ensheathed single central vascular strand gives off branches to the leaves 

 and is from time to time reinforced by additions from above. Subsequently it divides 

 dorsiventrally into two, in a manner which need not be described here, and the original 

 vascular tube having in the meantime become transformed into a complex tuliular network 

 of strands, the state of affaii's which is characteristic of the adult is reached. 



As the result of the observations described in the foregoing paragraphs, the writer 

 has reached the conclusion that the outer bundles are not cortical, as is stated by Van 

 Tieghem, and that the two large inner ones, which he appears to have confused with the 

 dorsal and ventral primary strands of the yovniger horizontal rhizome, are in reality medul- 

 lary strands. That the outer series is priiniti\'e is indicated, moreover, by the fact that Ijoth 

 the root-traces and the leaf-traces are attached to it. 



The above account gives little support to Van Tieghem' s theory of polystely, since 

 the young vascular axis is first and always a tube and does not become successively divided 

 into two, four, eight, etc., strands, as he describes. It might be supposed that the state of 

 affairs in Fteris aquUina is possibly abnormal, but the writer is in the position to assert, 

 from the examination of the development of a large number of vascular cryptogams, 

 belonging to the most different groups, that the course of development in Pfer'ts nquiii/ia 

 is quite typical, and that there is no evidence in an}' case which has come under his notice 



