SPORE-PRODUCING MEMBERS 457 
to distinguish the cells which will give rise to the sporangia (Fig. 255 8), 
but from rather older stages it appears that the sporogenous group, 
together with the superficial cells which cover it, is referable in origin to the 
segmentation of a single superficial cell (Figs. 255 Cc, D, E). Moreover, 
the first periclinal division of that cell defines the whole of the 
sporogenous tissue from the protective wall. As the sporangia grow older 
they project from the surface of the sporangiophore; the sporogenous 
mass increases rapidly in bulk, while the cells surrounding the sporo- 
genous mass, to the extent of several layers, assume the character of a 
tapetum (Fig. 255 G), which gradually becomes disorganised ; finally the 
sporogenous cells separate, and divide into tetrads.! As the sporangia 
approach maturity, the upper part of the sporangiophore may grow out 
into an irregular rosette of laciniae of vegetative tissue. These are seen 
in Fig. 244 G. 
Comparing the development of the sporangia in the three genera, it 
appears that with the larger size goes less definite segmentation, while greater 
definiteness is seen in the smaller types. It has been shown that the 
essential parts of the sporangium of Ophzoglossum cannot be referred in 
origin to a single cell, while those of Botrychium and Helminthostachys 
can in normal cases, Also, that the large sporogenous mass of Ofhzo- 
glossum throws off its superficial tissues as tapetum, which is of variable 
bulk:? in the other two genera the tapetum originates from the adjoining 
tissue, entirely outside the sporogenous mass. Further, when the definitive 
sporogenous tissue is clearly marked off, there is reason to believe that 
all the cells normally undergo the tetrad-division in all the three genera. 
The Ophioglossaceae may in fact be arranged in sequence, from those with 
large and indefinite sporangia to those with smaller and more definite, 
Helminthostachys taking the middle position. 
The same sequence emerges also from the comparative study of their 
fertile spikes. The least elaborated type is that of Ophioglossum, with its 
two series of sunken sporangia. Its spikes are liable in many species to 
occasional bifurcation, or even complete fission, and in O. pendulum and 
palmatum this may become habitual: but these are only cases of repetition 
of the same unelaborated part. In elminthostachys the external form as 
well as the development show that the sunken sporangia of Ophioglossum 
are replaced by sporangiophores, with separate and smaller sporangia, 
which at the same time are more numerous. In Sotrychium the elabora- 
tion of the spike takes a different line: the occurrence of synangia has 
been shown to be closely related to the branching of the spike, which 
extends to a high degree, while the numerous separate and relatively 
small sporangia continue to hold the same relative position as in 
Ophioglossum. 
It is possible, as in the case of almost all organic sequences, to 
regard this series as either an upward one of progressive elaboration or a 
1Beer, Annals of Bot., xx., p. 177+ 2Stevens, Azz. of Bot., vol. xix., p. 472. 
