‘EMBRYOLOGY 351 
There remains the type of embryogeny of Z. cernuum,' shared in all 
essentials by Z. imundatum. WHeré the initial steps appear to be like 
those of other species, but the lower tier of cells which elsewhere forms the 
foot remains small, and as a body consisting of but few cells it serves to 
maintain a connection with the parent prothallus (Fig. 187). The upper tier 
as usual originates the several parts of the embryo: breaking through the 
prothallial tissue it emerges early as a free-growing structure; but it swells 
early into an undifferentiated tuberous body, the ‘“protocorm,” which is 
roughly spherical in form, composed exclusively of parenchyma, and attached 
to the soil by root-hairs. It is occupied by a symbiotic fungus. However 
similar to the swollen foot of the clavatum-type this ‘‘ protocorm” may 
be, it is essentially a body of different origin: the foot springs from the 
lower tier of the embryo, and remains intra-prothallial: the protocorm 
originates from the upper tier, and is extra-prothallial. It was at first 
regarded as a foot which had quitted the prothallus; but developmentally 
it is distinct, while there is no evidence that an escape of the foot from 
the prothallus ever took place. The protocorm must therefore be held 
to be a body different in origin and nature from the foot in the clavatum- 
type. The part of the “protocorm” directed upwards bears a conical 
papilla of tissue, which develops into a cylindrical cotyledon: this is a 
green assimilating organ, with or without vascular tissue: it is succeeded 
by other leaves of similar type, which are, however, indefinite both in 
number and in position (Fig. 188). Relatively late the apex of the axis 
is recognised: its position is described as being near to the latest formed 
leaf, and the subsequent leaves arise from it in the usual acropetal succession, 
thus constituting the normal shoot. Close to its base the first root is also 
formed, and thus the normal plant is at length established. 
The existence of a. tuberous stage, prior to the establishment of the 
normal sporophyte in these species, has given rise to Treub’s well-known 
Theory of the Protocorm, while the very similar structure which is found 
perpetuated, and annually repeated in the life of Phyloglossum, added 
interest to the question of the real nature of the tuber in Z. cernuum ; 
but before its nature is discussed, it will be well to describe the leading 
facts in Phylloglossum. The prothallus of Phyloglossum appears, from the 
description of Thomas,’ to be of the cermuum-type, but it resembles most 
nearly ‘that of Z. iaundatum: it has, however, no leaf-like assimilating 
lobes on the green crown, which projects above the soil. The archegonia 
appear upon the assimilating crown, and produce an embryo which is 
similar to that of Z. cernuum: it projects early from the prothallus, the 
cot yledon being the first part to emerge: this develops as a green assimilating 
leaf similar to those of subsequent years. A “protocorm” is formed at 
once below the first leaf, and apparently in the same manner as the 
adult plant forms its tuber. No root has been observed during the 
1Treub, Butt, Ann., viii., ps Ie 2Goebel, Bot. Zezt., 1887, p. 183. 
3 Proc. Roy. Soc., vol. \xix., p. 285. 
