4i 8 SPHENOPHYLLALES. B. PSILOTACEAE 



The same is the character of the more superficial cells of the sporogenous 

 mass (Fig. 232 D), so that it is almost impossible to recognise the limit 

 between the tissue of the wall and of the sporogenous mass : the superficial 

 portions of the latter become disorganised without the formation of spores, 

 but there is no clearly defined tapetum. Such is also the fate of a 

 considerable proportion of the more central cells : for as the synangium 

 develops, irregular groups of cells of the sporogenous masses assume 

 dense granular contents, and subdivide, while the others remain paler, 

 with more watery contents, and do not divide : the former undergo the 

 final tetrad-division and form spores, while the latter become disorganised. 

 The actual state of partial disorganisation is shown in Fig. 232 E: thus a 

 partial sterilisation of cells of the sporogenous tissue, essentially similar to 

 that in Equisetum^ is seen also in Psilotum, and, as above stated, it occurs 

 also in Tmesipteris. 



In both genera the lines of dehiscence at maturity are defined 

 structurally. In Psilotum the lines radiate from the centre, and it has 

 been seen that a similar condition may be recognised in the sporangio- 

 phores of Sphenophyllum majus. Thus, in the broad outlines of 

 structure, in function, and in position the sporangiophore of the Psilotaceae 

 is the correlative of that in the Sphenophylleae. It remains to see how 

 far there is a correspondence also in the anatomical structure of these 

 plants. 



ANATOMY. 



The internal structure of the Psilotaceae is relatively simple, in 

 accordance with their outward form. The axis is traversed by a stele, 

 which is fundamentally of the protostelic type, and limited by an 

 endodermis which is more definite in Psilotum than in Tmesipteris. The 

 broad cortex which surrounds the stele is variously differentiated in the 

 rhizome and in the aerial shoot : in the former it consists of starchy 

 parenchyma, with endophytic mycorhiza in the outer layers, while the 

 superficial cells grow out into rhizoids : in the aerial shoot the stele is 

 surrounded successively by thin-walled parenchyma, sclerenchyma, and 

 assimilating tissue, while peripherally is an epidermis with stomata. Such 

 characters, however, present little that is of comparative value ; it is in 

 the vascular tissue that a better basis for comparison is found. 



The structure of the stele in Psilotum^ varies according to the position 

 and size of the part : in the rhizome there is often no protoxylem, but 

 the xylem consists of a somewhat irregular, and exiguous group of 

 scalariform tracheides, surrounded by an ill-developed phloem, while peri- 

 pherally there is a definite endodermis. At the base of an aerial shoot 

 the xylem increases in bulk, with interspersed parenchyma cells, and 



1 These statements are based partly on personal observations, but also on the writings 

 of Bertrand (Arch. hot. du Nord., i.), of Boodle (Ann. of Bot., xviii., p. 505), and of 

 Miss Ford (Ann. of Bot., xviii., p. 589). 



