Secondary Xylem in Psilotum. 513 
reduction in consequence of its saprophytism. Dr. Scott, however, kindly 
gives me the information that organs with indeterminate xylem are some- 
times associated with specimens of Sphenophyllum , and may possibly prove 
to be the rhizome of that plant. 
Thus in structure the base of the stem of Psilotum recalls the stem 
of Sphenophyllum , while the upper part of the stem resembles the axis 
of the cone of Cheirostrobus , a plant with some relationship to the 
Sphenophylleae. 
This amount of structural agreement 1 owes its value to the fact that 
the sporophylls of the plants concerned show distinct relationship of type. 
The correspondence of the typical synangium of the Psiloteae ( Tmesipteris 
and Psilotum ), together with its pedicel, to the sporangiophore of the 
Sphenophylleae was pointed out by Scott (’ 97 , p. 37, and '00, p. 499), and 
this correspondence was found by Thomas (’ 02 ) to become, on the whole, 
still closer in the case of variations of the sporophylls of Tmesipteris to 
a presumably more primitive type 2 . The whorled arrangement of the 
leaves in the Sphenophylleae is a marked point of distinction from the Psilo- 
teae. This and the nature of the sporophylls and the other characters of 
these two orders are discussed by Bower (’ 03 , p. 227 et seq.), who adopts 
the hypothesis of the relationship, and further agrees with Thomas that 
the Psilotaceae should be included in the group Sphenophyllales. 
It must be mentioned here that Lignier (' 03 , p. 106, &c.) holds the 
view that the Tmesipterideae or analogous types are among the ancestors 
of the Sphenophyllales, but he intercalates a Filicinean type between the 
two. Lignier (’ 03 , p. 105) also assumes as probable the presence of 
secondary xylem and phloem in the ancient Filicinean types, so that on 
this supposition the presence of secondary thickening in living Psilotaceae 
and the deduction of its existence in their remote ancestors would not be 
discordant with Lignier’s hypothesis of the phylogeny. 
1 In view of the fact that modifications of vascular structure can readily take place, the value 
of such resemblances between a recent plant and an ancient type may appear very small, but that 
well-defined details of primary structure may be retained for long ages is generally admitted and is 
well illustrated by the case of Equisetum and the Calamarieae. The value of seco 7 idary xylem 
as a character must be admitted to be much less, especially as secondary thickening appears to have 
been so general among vascular plants in the coal-measures. In a case of this kind, however, every 
additional point of agreement has some value, and, further, the presence of secondary growth may 
be taken as suggesting affinity with ancient types of vascular cryptogams. As a parallel case 
fsoetes may be quoted, in which the presence of secondary thickening helps the view of affinity with 
the Lepidodendreae. In a genus like Isoetes, in which a large number of species are submerged 
aquatics, the presence of secondary thickening is rather surprising, and the same is the case in a less 
degree for Psilotum with its reduced transpiring surface. That is to say, in both cases there is 
a probability against the secondary growth being a recent acquirement. 
2 Namely branched sporophylls and formation of extra sporangiophores. Bower (’03, pp. 228, 
229) regards these as not reversional, but as cases of increased complexity due to favourable nutrition 
as the determining factor (in this respect agreeing with Thomas’s view), but corresponding to 
‘ morphological possibilities of further amplification.’ 
