Development of Sporangia upon Fern Prothalli. 259 



-groups of sporangia are present must be looked upon as a sympodium. 

 Some probability is lent to this view by the fact that the first- 

 appearance of the process in Lastrcea is usually as a sympodial con- 

 tinuation of the axis of a prothallus whose true apex has developed 

 one or more sporangia. 



Since the group of sporangia and the tissue of peculiar character 

 on which they are seated are developed in the place of an apoga- 

 mously produced vegetative bud, they may be looked -upon as con- 

 stituting a very reduced sporophyte. The drain upon the resources 

 of the prothallus entailed by the production of this reduced bud, 

 which is incapable of further growth, is much less than when a 

 vegetative bud is formed. This explains why a number of such 

 sporangial groups can be produced and supported by a single pro- 

 thallus. The occurrence of a number of vegetative buds on a single 

 prothallus is the exception, but may happen, as the case of Aspidium 

 frondosum, before mentioned, shows. 



It is probable that it is in the constitution of the nuclei that a 

 means of distinction between cells of the oophyte and the sporophyte 

 must be looked for in these cases in which the two generations are 

 in intimate connection with each other.* 



The complete life history of the fern is in these cases still further 

 shortened than in the ordinary cases of apogamy ; not merely the 

 formation of a zygote by the fusion of antherozoid and ovum, but the 

 formation of an embryo, in which any differentiation of the vegeta- 

 tive organs can be detected, is omitted, and the sporophyte is reduced 

 to a mass of tissue which may be compared to a placenta bearing 

 sporangia. The occurrence of single sporangia upon the edge of the 

 prothallus may, in the light of the series of stages described, be con- 

 sidered as a still further case of reduction of an apogamous sporo- 

 phyte. While this does not altogether prevent the explanation of 

 the presence of sporangia upon the prothallus from the point of view 

 of the supporters of the homologous nature of the two generations, 

 it brings the present case into line with other exceptions to the 

 normal life-history cycle, whose bearing on the nature of alternation 

 has been discussed by Bower, f The present case, although more 

 striking in its appearance, seems, so far as it has been investigated, 

 to afford no sufficient reason for dissenting from the conclusion at 

 which he arrived. 



It is of interest to note the additional evidence, were such needed, 

 which these observations afford of the generalization made by 

 Goebel,J that the sporangium is to be regarded as an organ sui 

 generis. 



* Bower, ' Trans. Bot. Soc. Edinb.,' vol. 20. 

 t ' Annals of Botany,' vol. 4, 1890, p. 347. 

 t ' Bot. Zeit.,' 1881, p. 707. 

 VOL. LX. X 



