604 



FILICALES 



of the spores produced from each sporangium. If in a sorus of the type 

 of Gleichenia dichotoma the receptacle were elongated to receive a basipetal 

 succession of sporangia, which retained their form, but showed a diminished 

 spore-output, and lateral dehiscence in accordance with their basipetal 



sequence, the sorus of Alsophila would be 

 the result. 



Such changes are inherently probable, 

 and it has been seen in the sorus of 

 the Hymenophyllaceae how the greater 

 number of sporangia goes along with a 

 fall in their individual productiveness. 

 This is carried further in Cyathea than in 

 Alsophila, for there the sporangia are 

 smaller, and the output in C. dealbata 

 may fall as low as 16, or even 8 spores 

 per sporangium, though in C. medullaris 

 the number may remain at 64. The 

 development of the sorus in this genus 

 has also been followed : it differs in no 



shows a very young sorus, with receptacle CSSCntial point from that of AlsopJllla, 



excepting in the presence of the basal 

 indusium, which appears before any of the 

 sporangia (Fig. 335). The inconstancy of 



occurrence of the indusium in a group of closely related plants indicates 

 clearly that, however large, or early in appearance, or biologically important 

 it may be, it is not to be held as an essential part of the sorus, nor trust- 

 worthy as a phyletic character. 



ANATOMY. 



Anatomically the Cyatheae show very great complexity of structure, 

 though it can be referred, even in the most complex examples, by comparison 

 to a simpler source : the conclusions are, however, rendered less certain 

 by the lack of graded intermediate conditions. A relatively simple state 

 was found by H. Karsten x in the western species Alsophila pruinata, a 

 Fern which grows with an upright stem some three feet or more in height. 

 In transverse sections of the axis a solenostelic structure is seen, which 

 opens here and there with a foliar gap, from the margin of which the 

 leaf-trace is given off, apparently as a simple strand, with the usual horse- 

 shoe-like transverse section r after leaving the axis the leaf-trace soon breaks 

 up into a number of strands. As the internodes are of perceptible length 

 the leaf-gaps do not overlap, and the stele often appears as a complete ring 

 (Fig. 336). A peculiar feature is seen in this Fern in the leafless 

 runners, which originate below the leaf-bases, and grow like roots downwards 

 into the soil : it is interesting to note that they have at first a solid stele, 



FIG. 335- 



Cyathea dealbata, Sw. The upper figure 

 lows a very young sorus, with receptacle 

 and indusium already indicated. The lower 

 shows the indusium (z) more advanced, and 

 the sporangia s, s, arising in basipetal 

 succession. X 200. 



1 Vegetationsorgane d, Palmeii., p. 123. 



