XV] ORIGIN OF PARTS OF THE EMBRYO 311 



special suctorial organ when it appears externally as such. That is not 

 always the case, and there is no ground for assuming the presence of an 

 haustorium if it is not clearly recognisable as a special organ." The fact 

 appears to be that tissues derived from the hypobasal tract are liable to 

 distension in various ways in relation to the transfer of nutriment from the 

 prothallus to the embryo, taking a rounded form, and projecting towards 

 the source of supply. When such a suctorial organ is clearly distinguishable 

 it may be designated a " foot." But it is not constant in occurrence, size, or 

 position for Ferns. The relative constancy of a foot in the highly standardised 

 Leptosporangiate embryo has given such swellings a morphological recog- 

 nition they do not deserve even in the Filicales taken as a whole, and much 

 less in the Pteridophyta at large. 



The most important feature of the embryogeny, next to the definition 

 of polarity and consequent establishment of the axis, is the initiation of the 

 first leaf, or cotyledon. It always arises from the epibasal hemisphere, and is 

 orientated definitely in relation to the axis : but its time of appearance may 

 vary, and this is closely related to the nutrition of the embryo, whether 

 autotrophic or by mycorhizic saprophytism. No Family of Ferns shows 

 greater variety in this than the Ophioglossaceae. In H elminthostachys , or in 

 Botrychiimi obliqimm and virginianum, we probably see a relatively primitive 

 state (Figs. 288, 289). The cotyledon arises from the epibasal hemisphere 

 side by side with the apex of the axis and, soon appearing above ground, 

 expands as the first photo-synthetic leaf Campbell and Lang have, however, 

 observed that in Heljiimtkostachys the cotyledon itself may sometimes remain 

 rudimentary, as are several of the early leaves o{ Botrychium Ltmaria : this 

 is probably a derivative state following on mycorhizic nutrition, and bringing 

 with it a delay in their initiation. A curiously contrasted modification has 

 been described for Ophioglossum peduitculosum and moluccanum{ii^). Here 

 the cotyledon appears early and rises at once above ground as a photo- 

 synthetic leaf, but it extends downwards directly into the first root, while 

 the stem-apex is arrested, and appears to develop no further. It is stated 

 that it is replaced later by an adventitious bud originating from the root. 

 This also is probably a derivative condition from that seen in Helmintho- 

 stachys. Such peculiarities may affect the time of appearance and the pro- 

 portions of the cotyledon, but not its position relatively to the axis and 

 other parts. 



Von Goebel has, however, pointed out an apparent difference in relative 

 position of these organs between the embryos of the Marattiaceae and the 

 Leptosporangiate Ferns (254, p. 994). In the Marattiaceae the position is 

 as in the Ophioglossaceae, with the axis and cotyledon directed upwards, 

 that is away from the archegonial neck. In the Leptosporangiate Ferns the 

 cotyledon is directed downwards, that is, it is on the side next to the 



