EMBRYOLOGY 351 



There remains the type of elnbryogeny of Z. cernnum? shared in all 

 essentials by L. inundatum.- Here 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-^^e 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 davatum- 

 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 Phylloglossum, 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 Phylloglossum appears, from the 

 description of Thomas, 3 to be of the cernuum-type, but it resembles most 

 nearly that of Z. inundatum : 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 

 sim ilar 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 

 ic e below the first leaf, and apparently in the same manner as the 

 idult plant forms its tuber. No root has been observed during the 



Buit. Ann., viii., p. i. 2 Goebel, Bot. Zeit., 1887, p. 183. 



3 Proc. Roy. Soc., vol. Ixix., p. 285. 



