XIl] 



PROTECTION OF THE SORUS 



235 



the blade ; in Helininthostachys irregular lobes are borne on the distal end 

 of the sporangiophore, forming an imperfect covering to the sporangia while 

 young; a somewhat similar, but much more efficient covering, is found in 

 Matonia, where the receptacle is continued upwards into a leathery, umbrella- 

 like indusium, completely covering the sporangia while young. On the other 

 hand, Botryopteris and Zygopteris, the Osmundaceae and Gleicheniaceae, and 

 most of the Marattiaceae and Ophioglossaceae, have no special protection 

 for their sporangia: this may be held as a primitive state. 



It is otherwise with the Schizaeaceae. The sporangia in all of them are 

 marginal. In Schizaea, Anemia, and Mohria a strong growth on the upper, 

 or adaxial, side just below the young sporangium forces it towards the lower 

 surface, and the resulting flap appears as a continuation of the leaf-surface. 

 It curls round and protects the sporangium (compare Figs. 2 1 3, 214). It is the 

 same in Lygodiuni, except that here the growth appears on both sides of the 

 sporangium, forming a sort of pocket (Fig. 235). As it matures the pocket 

 differentiates into an upper lip which appears as though it were a continuation 



Fig. 235. Origin of marginal, "monangial" sorus of LygoditiDi. ,•:/= margin of young 

 pinnule in surface view; w = marginal cells; jr/ = sporangium. j5 = a similar pinnule 

 in surface view; «/ = apex; ? = indusium. A and B after Prantl. C=the same seen in 

 section. After Binford. 



of the blade of the leaf-segment, and a membranous lower lip. The two 

 completely enclose the single sporangium. The name indusium has been 

 applied to these growths. That of Lygodiuni may be accepted as the proto- 

 type of the form of indusium which has prevailed along the whole of the 

 Dicksonioid-Davallioid-Pterid series, and it is found also in Loxsoma, and 

 the Hymenophyllaceae. In all of them the indusium is basal, more or less 

 distinctly two-lipped, and it arises as it does in all the Schizaeaceae by 

 growth of superficial, not of marginal cells. The development of the indusium 

 of this type, together with the abortion of the inner lip, has already been 

 discussed and illustrated above (p. 222). Its persistence in marginal and 

 gradate sori finds its natural explanation in the fact that a basal protection 

 is important for the youngest sporangia, which are the lowest on the recep- 

 tacle. It is when the marginal position passes into the superficial and the 

 sorus itself assumes the mixed character that the protection being no longer 

 essential, the lower indusium is abortive, while the upper may merge into 

 the leaf-surface. 



