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, Maraitia Douglasii, C, 

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

 Osmunda Claytoniana, E, Adlanlum emarginatum, representing Polypodi- 

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

 horsetail (Calamophyte) (.Equisetum ielamateia) ; H, a late gymnosperm 

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



sporophyte of the lower Uverwort 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^ of 

 fertile tissue. This progressive sterihzation is repeated 

 in the ontogeny of the sporophytes of the higher forms. 

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

 'See foot-note, p. 125. 



