Phylloglossum Drummondii , Kunze . 325 
The fact that a gap is caused in the main stele by the exit of the 
vascular strand of the tuber has been used as evidence for regarding the 
tuber of fertile plants as morphologically a branch (p. 322). It is therefore 
of the greatest importance to find that the gap is such a characteristic 
feature in the anatomy of fertile plants. 1 
Other evidence was found in the fact that the tuber stele of the plant 
first described makes a sharp bend as it passes out, thus suggesting such 
a change in the direction of growth of the organ as the branch theory of its 
morphology would involve (Fig. 4, A, /). In the majority of fertile plants 
this is not the case, for the tuber strand follows, almost as soon as it is free, 
a direct downward course, but stelar tissue is sometimes found above the 
level at which the tuber stele is given off (Fig. 5, sects. 2 and 3), and 
doubtless represents the definite bend seen in a better-differentiated plant. 
That the difference is chiefly one of degree may be seen by comparing the 
large tuber of Fig. 4, A, with the left-hand tuber of Fig. 4, B. 
In this connexion it is also important to note in Fig. 5 how the form 
of the tuber stele changes as it passes out. When first free, it consists in 
cross-section of a horseshoe-shaped mass of xylem, with the arms of the 
horseshoe turned towards the main stele (sect. 4), but by the time it enters 
the shaft of the tuber (sect. 6) the curvature is in the opposite direction. 
The two positions of the gap in this single strand correspond to the two 
gaps seen in Fig. 3, sect. 6, where the stele of the tuber is cut in two places 
below the bend. We have, therefore, additional evidence that the tuber is 
an organ which, in the course of specialization, has made a definite change 
in its direction of growth. It is interesting that features similar to the 
above are shown by the steles of several other well-developed tubers. 
Before leaving the subject of the course of the tuber stele, reference 
should be made to its behaviour as it traverses the shaft of the tuber. Not 
infrequently the tuber stele diminishes rapidly to a slender strand of 
tracheides, and dies out before the swollen end of the tuber is reached. In 
the larger specimens, on the other hand, the horseshoe form of the stele 
may be retained until it nears the swollen region, where it breaks up to 
form several separate strands. By further subdivision the tuber stele 
forms an irregular sheath of tracheides in the tissue surrounding the bud 
of the storage tuber. 2 These tracheides, which are little lignified and 
difficult to distinguish from the cells of the ground tissue, die out before 
the base of the bud is reached. Their presence in the largest tubers is 
1 The possibility of the gap being foliar, which was suggested by Wernham, is rendered still 
less probable by recent work, which dissociates very completely the Ophioglossaceae from the 
Lycopods. Tniesipteris , even if the stelar gaps are foliar, which is doubtful, is more commonly 
regarded now as showing Sphenopsid rather than Lycopsid alliance. Moreover, the * organe de 
Mettenius which Wernham suggests may be the vestige of a megaphyllous leaf is in reality a stage 
in the reduction of one of the normal leaves of the plant (p. 326). 
2 This sheath of tracheides was described and figured by Bertrand, loc. cit., 1885. 
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