COMPARATIVE ANATOMY OF THE CYCADACEE. 111 
and Macrozamia. Like the similar cases in these three plants, the extrafascicular 
tissues occur only outside a small part of the circumference of the central stele. 
Moreover, and this is the most important point of all, these outer vascular tissues have 
their parts diversely orientated—that is to say, that a, ‘ion of the strand, viz. the 
innermost portion, is inversely orientated, with phloem u. :cted inwards and xylem 
outwards. ‘This same phenomenon has been shown to occur in the region of the primary 
node of the three genera mentioned above. But, in these latter, it was seen that the 
strands concerned assumed the form of more or less perfect concentric structures, having 
a continuous or almost continuous elliptical contour; they are thus comparable to the 
concentric strands of Medullosa Solmsii, Schenk, var. typica, represented on pl. 3. 
figs. 1-4 of Weber & Sterzel’s paper *. In the plant under consideration, however, the 
compound strand, consisting of the normally-orientated, with its opposed inversely- 
orientated, portion, is observed in the form, not of a concentric structure, but of a 
flattened plate, and is thus exactly comparable and homologous with the single flattened 
* Schlangenring" of Medullosa porosa, Cotta. I have observed a structure similar to 
this at one place in the circumference of the first extrafascicular ring of a seedling of 
Cycas revoluta, Thunb.t+, where the inner inverted portion of the strand was, however, 
quite indistinct and its tissues but weakly developed as compared with the opposed 
normally-orientated portion. In Bowenia, on the contrary, at the young stage of 
development to which the extrafascicular ring has attained, the inner inverted portion is 
much more advanced than the outer (Pl. 15. fig. 2). As the plant grows older the latter 
will, doubtless, supersede and develop greatly in advance of the former, which will 
eventually, as in the older stems of the three genera above mentioned, become obliterated 
and be no longer recognizable. 
. Thus we see the young plant, in the preliminary stages of its development during the 
period when secondary thickening arises in the axis, exhibiting ancient ancestral phases 
of character which yield us a clue toits descent. Such ancestral rudimentary structures, 
conspicuous and pronounced in the young organism, but becoming obscured or completely 
vanishing in the adult, biologists have long been familiar with both in the animal and 
vegetable kingdoms. 
The first-formed tracheides of the extrafascicular strand (fig. 2, 7?) are homologous 
with the same, short, isodiametric, reticulately-thickened tracheides occurring scattered 
promiscuously in the centre of all ihe secondary concentric strands in Cycas and the 
other genera. 
The extrafascicular tissues were observed not merely in a single plant belonging to 
the batch emanating from St. Albans, but also in an individual kindly sent to me by 
Mr. W. G. Groves, of Holehird, Windermere, which belongs to the type form with 
acinaciform leaves, and which exhibits the tissues in question extending for a greater 
distance round the circumference of the central stele than is the case in the other plant. 
In Bowenia we see, therefore, a fourth genus of Cycadacee exhibiting the phenomenon 
of extrafascicular vascular tissues in the axis. i 
* Beiträge zur Kenntniss der Medulloseæ, 1896. 
+ Journ. Linn. Soc., Bot. vol. xxxiii. 1898, p. 441. 
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