Page:Proceedings of the Royal Society of London Vol 60.djvu/285

Rh groups of sporangia are present m ust be looked upon as a sym podium. Some probability is lent to th is view by th e fact th a t th e first appearance of the process in Lastrcea is usually as a sym podial continuation of th e axis of a prothallus whose tru e apex has developed one or more sporangia.

Since the group of sporangia and th e tissue of peculiar character on which they are seated are developed in the place of an apogamously produced vegetative bud, they m ay be looked upon as constituting a very reduced sporophyte. The drain upon the resources of the prothallus entailed by the production of this reduced bud, which is incapable of fu rth er grow th, is m uch less th an when a vegetative bud is formed. This explains w hy a num ber of such sporangial groups can be produced and supported by a single p rothallus. The occurrence of a num ber of vegetative buds on a single prothallus is th e exception, but may happen, as th e case of frondosum, before m entioned, shows. It is probable th at it is in the constitution of th e nuclei th a t a means of distinction between cells of the oophyte and the sporophyte m ust be looked for in these cases in which th e two generations are in intim ate connection w ith each other.*

The complete life history of the fern is in these cases still fu rth e r shortened than in the ordinary cases of apogam y; not m erely the formation of a zygote by the fusion of antherozoid and ovum, b u t the formation of an embryo, in which any differentiation of th e vegetative organs can be detected, is om itted, 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 th e edge of the prothallus may, in the light of the series of stages described, be considered as a still fu rth er case of reduction of an apogamous sporophyte. 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 biings the present case into line w ith other exceptions to the normal life-history cycle, whose bearing on the n atu re of alternation ias been discussed by Bower, f The present case, although more striking m 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 °e el,+ th at the sporangium is to be regarded as an organ sui generis.

t * Annals of Botany,’ vol. 4, 1890, p. 347. X ‘ Bot. Zeii;.,’ 1881, p. 707.
 * Bower, ‘ Trans. Bot. Soc. Edinb.,’ vol. 20.