THEORY OF THE PROTOCORM 225 



sporophyte on the soil. Such a suggestion certainly accords readily with 

 the sporadic occurrence of the " protocorm." 



It is difficult to arrive at a conclusive balance between such conflicting 

 facts and arguments as these. So far as any conclusion commends itself 

 to my mind it is as follows : A " protocorm " development may have 

 been an important phase in the establishment of certain Lycopod embryos, 

 in that it serves as a temporary substitute for a root-system delayed in 

 its development. But it seems unnecessary to take such cases as proto- 

 types for even the genus Lycopodium as a whole : since the Lycopod 

 embryo, while showing essential unity in its general plan, seems prone 

 to parenchymatous swelling. Two such swellings, somewhat similar in 

 structure but differing in place of origin and in function, are known, viz., 

 the enlarged "foot" of L. davatum and annotinum, which originates 

 from the lower tier of cells of the embryo, and is intra-prothallial ; and 

 the " protocorm " of the cernum-type, which originates from the upper 

 tier of the embryo, and is extra-prothallial. They are both biologically 

 intelligible, for the former acts as an haustorium, the latter may be a 

 ready mode of fixation in the soil, and also a specialised place of storage. 

 A genus which shows two types of parenchymatous swellings in two 

 distinct types of embryo, while both are absent from other species of 

 the genus, cannot be expected to have ever had one of these as a 

 constant feature in its ancestry. This consideration makes me doubt 

 any general application of the theory of the " protocorm " even in the 

 genus Lycopodium. These parenchymatous swellings may be looked upon 

 as opportunist growths, rather than as persistent relics constant from a 

 remote ancestry. This view is greatly strengthened by the occurrence of 

 protocorm-like developments in isolated cases among the Angiosperms. 

 Phylloglossum with its large storage " protocorm " would then be the 

 extreme type of a line of embryological specialisation, not a form pre- 

 serving the primitive embryological characters of the whole race. On 

 such grounds, while not denying that a " protocorm " may have had a 

 certain importance in certain cases, the facts do not appear to justify 

 attaching to it any general significance. 



From the above pages it will be plain that the origin of the free- 

 living habit of the sporophyte, and of its root-system is quite as obscure 

 as that of the leafy shoot itself. The important step from dependent to 

 free life was certainly taken at a period before the earliest fossil records of 

 Vascular Plants ; for all the best-known types of early fossil Pteridophytes 

 have roots assigned to them on secure grounds of observation : so 

 naturally the evidence from them does not lead to a solution of the 

 difficulty. On the basis of comparison, to which this question must 

 necessarily be relegated, no decisive help is forthcoming ; the -theory of 

 the protocorm, which at first sight seemed so full of promise, does not 

 give more than a suggestion how the transition from dependence to 



independence may actually be carried out in certain cases, and among 



p 



