FERN CROSSING AND HYBRIDISING. 293 
so hybridised, to withstand low temperature, due to the infusion of hardy 
blood. In Ferns, thanks to the curious fact that the prothallus, or green 
scale upon which the flower homologues are produced, is almost constant 
in size throughout all species except the Filmies, the minutest species — 
B A+B A 
Fra. 109. —PonysticHuUM ANGULARE PoLypacryLa (B) x P. A, LINEARE (A). 
Resutt or THE Cross (A+B). 
The x marks indicate defects in parent and progeny. 
and the largest Tree Fern commence their career under practically iden- 
tical conditions, and crossing and hybridisation therefore are not limited 
as in flowers by incompatibilities between length of stigma and size of 
pollen. Hence, so far as size is concerned, there is no bar, and the 
. 
