126 



HEREDITY AND EVOLUTION IN PLANTS 



101. Evidence from Comparative Ontogeny. In zool- 

 ogy, evidence of the course of evolution is also seen in the 

 recapitulation of the characters of lower forms in the em- 

 bryogeny of higher forms. This is often referred to as von 

 Baer's law. Evidence of that nature is less striking and 

 less common in plants. It is found, however, in a com- 

 parison of the young or embryonic stage of the sporophyte 

 of the higher liverwort, Marchantia, with the mature 



FIG. 64. The apical cell in the stem apex in various phyla, from 

 Bryophytes to Gymnosperms. A, acrogynous liverwort (Notothylus 

 orbicularis)', B & C, eusporangiate ferns (B, Marattia Douglasii, C, 

 Ophioglossum pendulum)', D & E, homosporous leptosporangiate ferns (D, 

 Osmunda Claytoniana, E, Adiantum emarginatum, representing Polypodi- 

 ales); F, heterosporous leptosporangiate fern (Marsilia vestita)', G, a 

 horsetail (Calamophyte) (Equisetum telamateia) ; H, a late gymnosperm 

 (Pinus Laricio). (A-G redrawn from Campbell, H from Buchholz). 



sporophyte of the lower liverwort Riccia (Fig. 65). The 

 latter consists almost entirely of "fertile" (i.e., reproduc- 

 tive) cells. As we pass to more highly organized forms, 

 such as Marchantia, the relative amount of vegetative 

 tissue gradually increases by a progressive sterilization 1 of 

 fertile tissue. This progressive sterilization is repeated 

 in the ontogeny of the sporophytes of the higher forms. 

 The thread-like, green protonema of mosses is often in- 

 1 See foot-note, p. 125. 



