i68 COOK 



changes as apospory. Recapitulation affords an idea and 

 method of expression often very convenient for describing in a 

 general way the facts of developmental history, but evidently 

 does not represent a universal or causal principle or a necessary 

 sequence in evolutionary progress. 



In the archegoniate plants there are numerous independent 

 instances of what might be called syncopation or anticipation, 

 rather than recapitulation, for instead of merely following over 

 the ancestral lines of development in gradually shorter and less 

 accentuated form, various pieces of the life-history appear to 

 have dropped out, suddenly and completely. Thus in apospory 

 a cell which might normally give rise to spores, produces, in- 

 stead, a prothallus, such as might grow from the spores, which 

 are then entirely omitted from the program. In a similar 

 manner, the conjugation of two of the unspecialized cells of the 

 prothallus may result in the growth of a sporophyte, the forma- 

 tion of specialized archegonia, antheridia and gametes being 

 entirely omitted. Nor do these instances show the limits of 

 anticipatory abbreviation of the life-histories of the archegoniate 

 plants. 



Instead of merely producing prothallia by apospor}', many 

 ferns produce young plants by direct growth from the cells of 

 the leaves. Likewise in Isoetes^ young plants may replace the 

 sporangia, in the axils of the leaves, though it is possible that 

 the thread or column of cells on which these aposporous plantlets 

 are borne may represent the prothallus. Indeed, it may be that 

 IsoeteSy rather than the terrestrial heterosporous archegoniates, 

 represents the nearest pteridophytic analogue of the flowering 

 plants. Many of the smallest and least specialized angiosperms 

 are aquatic types, like the Lemnacea; and the Podostemonacea;, 

 and some of them have little more differentiation of tissues than 

 Anthoccros itself. 



Podostenion and its aquatic allies may never have had the 

 acrogenous internodal structure of the higher angiosperms, and 

 may illustrate a different method by which angiosperms could 

 arise from an Anthoceros-\\\f^Q. ancestor. The so-called leaves 

 of the Podostemonaceae appear much more analogous to tlie 

 fronds of ferns and algae than to the leaves of other angio- 



