480 OPHIOGLOSSALES 
On this view the two genera, starting from a common source, would be 
held to illustrate two distinct lines of progression to a more complex state. 
The third genus, with its single highly elaborate species, gives no such 
suggestion of its origin by comparison of nearly allied species. It stands 
as the most isolated member of the family: but its normal spike is 
evidently similar in plan to that of a large Ophzog/ossum, supposing its 
marginal rows of sunk sporangia were replaced in Aelminthostachys by 
serried ranks of sporangiophores. There is a biological probability that 
such an advance should occur in a large spike bearing many spores, for 
thereby the advantage is gained of more ready nutrition of the subdivided 
sacs, and more easy dissemination of the spores when mature. 
The progressive advance thus suggested for the Ophioglossaceae is in 
accord with biological probability, in a series with a marked tendency to 
a monophyllous state, and consequent enlargement of the individual leaf. 
Provided the nutrition be available, an increase in numbers of spores is 
an advantage in any homosporous form. But an indefinite increase in 
size of individual sacs raises difficulties of nutrition: subdivision is thus 
to be anticipated in any progressive series, and that is seen in Ophio- 
glossum. A projecting position of the individual sporangium is an 
advantage in dissemination of the spores. This is ill provided for in 
Ophioglossum, and in this respect Botrychium and Helminthostachys show 
a more effective state.. It appears to me difficult, without special reasons 
assigned, to recognise this family as a series of reduction, for it would 
be in opposition to these biological considerations. On the other hand, 
comparisons within the family clearly indicate an upward rather than a 
downward progression, while in any case those who hold a theory of 
reduction would find peculiar difficulty in explaining the condition seen 
in Ophioglossum palmatum., 
The next step wilf be to discuss the morphology of the fertile spike, 
and to see what are its nearest correlatives among the members of other 
Vascular Plants. The spike in all the representatives of the family is 
clearly the same part: it is in fact truly homologous, or homogenous in 
the strict evolutionary sense. This follows from the high degree of 
constancy of position and function which it shows in normal cases. 
Various theoretical explanations of its morphological nature have been 
given by different writers. It has been suggested by Braun! that the 
sterile frond is a foliage leaf, and the fertile spike the only developed 
leaf of a bud seated in its axil, and coalescent with it. Somewhat later, 
Roeper (1843) published the opinion that the sterile spike and fertile leaf 
are equivalent—that is, borne by the same axis—but coalescent together. 
Subsequently he substituted for his old view the opinion that the fertile 
spike is the result of coalescence of two lateral, lower, fertile pinnae of a 
frond, of which the remainder is usually sterile.2 Lastly, Goebel has put 
forward the opinion that the fertile spike is the lowest pinna of the 
1 Flora, 1839, p. 301. * Bot. Zeit., 1859, p. 271. 
