356 FERNS 



the fern-embryo has attained a degree of differentiation far 

 beyond anything which occurs in the moss-embryo. The 

 scarcely differentiated polyplast has passed into a stage 

 which may be called the phyllula, distinguished by the 

 possession of those two characteristic organs of the higher 

 plants, the leaf and root. 



Notice how early in development the essential features of 

 animal or plant manifest themselves. In Polygordius the 

 polyplast is succeeded by a gastrula distinguished by the 

 possession of a digestive cavity : in the fern no such cavity is 

 formed, but the polyplast is succeeded by a stage distinguished 

 by the possession of a leaf and root. In the one case the 

 characteristic organ for holozoic, in the other the character- 

 istic organs for holophytic nutrition, in the higher organisms, 

 make their appearance, and so mark the embryo at once as 

 as animal or plant. We may say then that while the oosperm 

 and polyplast stages of the embryo are common to the 

 higher plants and the higher animals, the correspondence 

 goes no further, the next step being the formation in the 

 animal of an enteron, in the plant of a leaf and root. In 

 other words the phyllula is the correlative of the gastrula. 



The cotyledon increases rapidly in size, and emerges 

 between the lobes of the kidney-shaped prothallus (L) : the 

 root at the same time grows to a considerable length, the 

 result being that the phyllula becomes a very obvious 

 structure in close connection with the prothallus, and indeed 

 appearing to be part of it. The two are actually, however, 

 quite distinct, their union depending merely upon the fact 

 that the foot of the phyllula is embedded in the tissue of the 

 prothallus like a root in the soil. Hence the phyllula is 

 related to the prothallus in precisely the same way as the 

 sporogonium to the moss plant (compare Fig. 84, K, with 

 Fig. 82, c 2 , and Fig. 84, L, with Fig. 82, c 4 ). 



