THE GENUS EQUISETUM. 



159 



tissue, the latter communicates periodically also with the fundamental tissue outside the 

 tubular axis. Tlie writer has not been aide to discover here any indication of the repeated 

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



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

 from the very first, a, hollow tubular librovascular axis. 



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

 which up to the present has grown perpendicularly 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 spiral 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 (ibrovascular 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 brown sclerenehyma, oval in transverse section, has made 

 its appearance in the midst of the fundamental tissue occupying the center of the stela r 

 tube. 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 oft' a bundle 

 into its cavity, which is quickly surrounded by a tubular sclerenchymatous sheath, formed 

 by the bending round it from below, 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 

 dorsi \entrally 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 tubular network 

 of strands, the state of affairs which is characteristic of the adult is reached. 



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

 lias 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 younger horizontal rhizome, are in reality medul- 

 lary strands. That the outer series is primitive is indicated, moreover, by the fact that both 

 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 Pteris aquilina 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 Pteris aquilina 

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



