414 SPHENOPHYLLALES. B. PSILOTACEAE 
similar are they in their initial stages, though so different when mature. 
Those, however, which are to develop as sporophylls soon show an 
increase in thickness, while they grow less in length; an excrescence of 
the adaxial surface soon becomes apparent (Fig. 230 A), in which the 
superficial cells are chiefly involved: this constitutes the sporangiophore. 
The superficial cells at first form a rather regular series: they undergo more 
a a 
sS ‘3a S Zi 
Se 
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Fic. .230. 
Tmesipteris tannensis. A=very young synangium arising from the adaxial surface 
of the sporophyll. xz00. =a sporophyll bearing a much older synangium ; the sporo- 
genous masses are shaded. X100. C=part of a radial section of a mature synangium 
showing the insertion of the septum. x 100. 
or less regular divisions: a band of tissue some four or more layers in depth 
is thus produced. At about this period certain masses of cells assume 
the characters of a sporogenous tissue (Fig. 230 B); but though they can 
be recognised as such by the character of the cells, it is exceedingly 
difficult to define the actual limits of these sporogenous masses. The 
more superficial tissues, as well as the band intervening between the two 
sporogenous masses remain sterile, the latter developing into the septum, 
while the former develop into the walls of the synangium: it is specially 
