EVOLUTION OF ANGIOSPERMS THROUGH APOSPORY 169 



sperms, and appear to be mere thalloid outgrowths of the priini- 

 tive capsule-base, like the fronds of ferns, instead of developing 

 as an acrogenous series of vegetative capsule-units. 



Systems of botanical classification usually treat the Podoste- 

 monace££ as degenerate representatives of one or another of the 

 terrestrial families, but the justification for this is not obvious. 

 That the family may prove to have historical significance as a 

 primitive group of angiosperms is suggested by some Paleozoic 

 fossils to which my attention has been called by Mr. David 

 White. These remains have been looked upon hitherto as 

 ferns, but Mr. White has ascertained that they produce quadri- 

 valvate capsules each apparently containing a single seed-like 

 body. The resemblance to some of the recent Podostemonaceae 

 is very striking throughout. Indeed, it is not now possible to 

 specify any differences by which they can be excluded from 

 that group, though Mr. White assures me that paleontologists 

 will be very reluctant to admit the existence of even very primi- 

 tive angiosperms in an epoch so remote.^ 



ELIMINATION OF MACROSPORES BY APOSPORY. 



If the present view can be justified, it will not be necessary 

 to think of the egg-cell or of the egg-apparatus as representing a 

 macrospore which remains attached to the parent plant and 

 germinates, instead of falling off as macrospores should. The 

 attachment of the young plant to its parent is adequately 

 explained by apospory, and can be viewed as entirely parallel 

 with the attachment of a fern-capsule to its parent plant or of 

 the very young fern-frond to the female prothallus from which 

 it springs. The egg-apparatus can be looked upon jis the arch- 



' The only obvious difference jet detected between these Palaeozoic fossils and 

 the modern Podostemonaceoe lies in the fact that the capsules of the latter are 

 many-seeded, while the supposed seeds of the fossils are single, and much larger. 

 Such diversities in the numbers and sizes of seeds inside the same familv are 

 very frequent, of course, among the flowering plants. The habits of life of the 

 Podostemonaceae would render many small seeds preferable to fewer large ones, 

 so that a reduction in size and an increase in numbers might naturally be ex- 

 pected as a result of a long evolutionary opportunity such as any extant de- 

 scendants of these paleozoic plants would have had. They are aquatic plants, 

 living in tropical water-courses, but the rocks over which they creep are often 

 completely exposed tor long periods in the dry season. The seeds have the 

 primitive function of carrying the species through the annual period of drought. 



