ITS BIOLOGICAL ASPECT 181 



till it is established in the soil. And as the prothallus differs in form and 

 position, in size and in duration of life, so the germ itself may differ in the 

 place and time of origin of its parts, as well as in their form and structure. 

 A few illustrations will show how this point of view gradually asserted itself. 



In 1882 a comparative revision of those parts which serve as haustoria 

 in various embryos led me to the conclusion that they are not to be regarded 

 as clearly denned morphological members, but rather as swellings of the 

 hypocotyl, which arise only where they are required for the first processes 

 of development and nutrition of the young embryo. 1 Some years later 

 Treub introduced his theory of the " Protocorm " :- theoretical considera- 

 tions of the biological condition of the young embryo had led him to 

 conceive of an organ preceding in descent the leafy shoot, such as is now 

 seen in Vascular Plants ; and this he recognised as actually present in the 

 embryonic tubercle of certain Lycopods : a preliminary stage, in fact, which 

 is to them as the protonema is to a Moss. Whatever view we may now 

 hold of the protocorm, this theory takes its place as a further step towards 

 a biological rather than a purely formal study of embryology. At the hands 

 of various other writers such views have been further developed, especially 

 in relation to the better knowledge recently acquired of the embryology 

 of the Lycopods and Ophioglossaceae : and it was thus open to Goebel to 

 formulate the position, as he does in his Organography? Having shown 

 that external forces do not come into consideration in the arrangement in 

 space of the parts of the embryo, 4 he points out that we need only consider 

 internal factors, and say generally that root, shoot, and haustorium are laid 

 down in the positions that are most beneficial for their function. This 

 is in fact an extension to the whole embryo of the conclusion which I 

 had applied in 1882 to the foot only. Comparative embryology of the 

 sporophyte would thus become essentially a study of the circumstances 

 and conditions which influenced the embryo during its evolution, and of 

 the way in which the germ is formed to meet them. 



But it may be enquired whether the germ itself does not still show 

 beneath these adaptive modifications, some characters of a central type ? 

 Is all trace of the early evolutionary history eliminated by the subsequent 

 modifications? There is at least one leading feature which remains traceable 

 with some degree of constancy throughout the series of known embryos 

 of the Pteridophyta : it is found in the relation of the parts to that initial 

 polarity which is established at a very early stage in them all. This may 

 often remain obscured owing to the precocious development of certain 

 parts, in response to biological requirements ; but nevertheless, it will be 

 shown as the several embryos are described, that the apex of the axis has 

 constantly a position in close relation to the intercrossing of the octant- 



1 Quart. Jottrn. Alicr. Sci., xxii., p. 277. 



- Bitiiiiizorg Annals, viii., p. i. The Theory of the Protocorm will be discussed at 

 length below, in relation to the embryogeny of the Lycopods. 



* Organograpky, ii., p. 246. 4 Ibid., i., p. 219. 



