PTEROPSIDA l6l 



but Dicksonia must clearly be removed on account of its 

 marginal sori, for the Cyatheaceae, as now constituted, have 

 superficial sori. The earUest known fossil representative of 

 the group is ^/^o;?/;////^^ from the Jurassic. Bower^ recognized 

 three Uving genera within the family: Alsophila with about 

 300 species, Hemitelia with about 100, and Cyathea with 

 about 300. 



Although the largest may attain a height of 25 m, some 

 species are comparatively low-growing. Much of the dia- 

 meter of the trunk is composed of matted adventitious roots 

 and persistent leaf bases, while the stem within is relatively 

 small. Nevertheless, its stelar anatomy is highly complex for, 

 in addition to a convoluted dictyostele, there are abundant 

 medullary strands, and sometimes cortical strands too. 

 Broad chaffy scales form a dense covering over the stem 

 apex and the base of the frond. 



The stipe receives a number of separate leaf-traces from 

 the lower margin of the associated leaf gap. While the fronds 

 of most species are several times pinnate, those of Cyathea 

 sinuata are simple. The venation is open in the majority of 

 species, except for very occasional vein fusions. 



The three genera recognized by Bower are distinguished 

 by the character of the indusium but, otherwise, the sori 

 are very similar in their gradate development. In Alsophila 

 there is no indusium at all, in Hemitelia there is a large 

 scale at one side of the receptacle, and in Cyathea (Fig. 25H) 

 it extends all round the receptacle to form a cup which com- 

 pletely covers the globose sorus when young, but which 

 becomes torn as the sporangia develop and push through it. 

 Holttum,^^ however, regards this distinction between the 

 three genera as artificial, and prefers to merge them into the 

 one genus Cyathea. Furthermore, he has recently changed 

 his opinion as to the affinities of the family, for he now 

 draws attention to the close similarity between the scale- 

 like indusium of some species and the lower indusium of 

 Dicksonia.'^^^ * 



