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 





FIG. 



230. 



Tmesipteris tannensis. /4=very young synangium arising from the adaxial surface 

 of the sporophyll. X 100. .5 = a sporophyll bearing a much older synangium ; the sporo- 

 genous masses are shaded. X 100. 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 



