54 



ALTERNATING GKNKRATIONS 



1-unaria, Brizi), 1 and from various species of Ferns belonging to the 

 Hymenophyllaceae and Polypodiaceae,- but no examples are on record 

 from the Lycopodiales or Equisetales. Those cells which would in the 

 normal course produce the spores take no part in the formation of the 

 gametophytic growths. In Anthoceros the origin of these is commonly 

 from sub-epidermal cells: in the Mosses from the cells of the seta, or of 

 the sporogonial wall : while in Ferns the archesporial cell if already denned 

 in a sporangium is abortive. Thus the aposporous growths are in no 



sense mere irregularities of 

 development from sporogenous 

 cells. In Anthoceros each 

 growth is apparently referable 

 in origin to a single cell, and 

 the same is probably the case 

 also for Mosses. But in the 

 Ferns this is not so : here the 

 vegetative development may 

 start from a sporangium formed 

 in its normal place : a plurality 

 of the cells of the stalk, or 

 of the sporangial wall surround- 

 ing the abortive central cell 

 may divide, and assume pro- 

 thalloid characters (Fig. 37), 

 or the growth may arise from 

 the receptacle of the sorus 

 (Fig. 37 E) : or again, it may 

 be initiated at some point 

 on the leaf, usually marginal, 

 which thus extends directly 

 into the prothallial expansion. 

 ;m<l may bear antheridia and 

 archegonia (Fig. 37 B, c, D). 



The matter may be further 

 complicated by the combination 

 of apogamy and apospory in the same individual, and this condition has 

 lu-rn seen in about half the recorded cases of these abnormalities in Ferns. 

 The apogamous seedlings of Nephrodium pseudo-inns, var. iris fata (Cropper), 

 not only sprang themselves in an apogamous manner from the prothalli, 

 but proceeded almost at once to an aposporous production of new prothalli 

 on the margins of the young leaves. :; These prothalli bore antheridia, 



1 Ann. ln*t. /'<'/. A'of//., v. . p. 54. 



-I'm- references see Kngler and I'rantl, Nat. /'//., i. 4, p. 88, and Goebel, /7<w, 

 1905, p. 239. 



I )iuci y, Jumn. J.inn. .S<;,., vol. xxix., p. 479. 



FIG. 36. 



Scolopendrivm 7'iilgarc. Group of sporangia (jy>) DM .1 

 ;:'(! ii ill' 1 1 111 ture of which indicate-- its relation to an 

 ii :;onium. Occasionally two nuclei are present iti a s 

 X6oo. (After 1,,-nm. ) 



