170 EMBRYOGENESIS IN PLANTS 



In lycopods, SeJagiiieUas, Equiseta and ferns, a leaf is usually formed 

 early in the embryogeny, though in some instances it may be delayed; 

 there may also be a delay in the organisation of the shoot apex. These 

 parallel cases of delay in organ formation appear to be referable to 

 similar nutritional relationships between embryo and prothallus. No 

 doubt they have a genetical basis, Psilotum and Tmesipteris are unique 

 among pteridophytes in that no leaf or root is formed during the 

 embryogeny, though older plants of Tmesipteris form scale-leaves and 

 then vasculated leaves. Whether Psilotum is a leafless and rootless 

 plant ab initio, or whether these organs were once present in the race 

 and have been eliminated during descent, is not definitely known, 

 though the balance of evidence has been considered by many morpho- 

 logists to support the former view. In the highly modified, aquatic, 

 heterosporous fern genus Salvinia, a rudimentary root is said to be 

 discernible in the early embryogeny but becomes merged in the 

 parenchymatous foot as the embryonic development proceeds. 



The Filicineae differ from all the other classes of Pteridophyta in 

 their possession of large or megaphyllous leaves, in contrast to the small 

 or microphyllous leaves of lycopods, Selaginellas, and horsetails. If 

 only the early embryogeny were considered, however, it is doubtful if 

 this distinction could be maintained: in respect of their origin and 

 positional relationship to the shoot apex, the first leaves of micro- 

 phyllous and megaphyllous types are not unlike. As the sporophyte 

 enlarges, however, and forms its second, third, and fourth leaves, the 

 difference between the two foliar types becomes very conspicuous. 

 Some observers consider that there is a fundamental difference between 

 the megaphyll and the microphyll (Bower 1935; Hamshaw Thomas, 

 1932), i.e. that they are not, or are probably not, truly homologous 

 organs. The present writer does not subscribe to this view. It is true 

 that megaphyllous species have a capacity for leaf-growth that is not 

 shared by microphyllous species; but in all the basic organogenic 

 relationships — formation near the basiscopic margin of the apical 

 meristem, dorsiventrality, vascularisation and phyllotaxis — there is no 

 fundamental difference between megaphylls and microphylls : they are 

 alike in being lateral foliar members, formed at the shoot apex. 



From this discussion, no clear view emerges as to the relationship 

 between the ferns and other pteridophytes. That their patterns of 

 embryonic development show many similarities is evident, but whether 

 these similarities are due to community of origin, or whether they are 

 to be regarded as parallelisms of development, cannot be decided on 

 the evidence thus far available. From the embryological data it appears 

 that megaphyllous and microphyllous groups, though quite distinct, do 

 not necessarily exemplify fundamentally different kinds of organisation. 



