SPORE-PRODUCING MEMBERS 457 



to distinguish the cells which will give rise to the sporangia (Fig. 255 n), 

 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 c, r>, 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 <;), which gradually becomes disorganised; finally the 

 sporogenous cells separate, and divide into tetrads. 1 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 c.. 



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 Ophioglossum cannot be referred in 

 origin to a single cell, while those of Botrychiitm and Helminthostachys 

 can in normal cases. Also, that the large sporogenous mass of Ophio- 

 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 Helminthostachys 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 Botrychiitm 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 



1 Beer, Annals of Bot., \\., ]>. 177. 2 Stevens, Ann. of Bof., vol. \i\., p. 472. 



