THK EVOLUTION OF PLANTS 223 



seeds is to be interpreted as a reversion to a more primitive 

 condition (Fig. 105).^ 



147. Origin of Monocotyledony. — If the earliest Angio- 

 sperms were dicotyledons, as now seems probable, the 

 monocotyledons were probably derived from them by 

 a process of simplification. Several hypotheses have been 

 framed as to how the final result was accomplished, but 

 the voluminous evidence and the conclusions can only be 

 briefly summarized here. 



For nearly a century it has been generally accepted 

 by botanists that the two seed-leaves or cotyledons of 

 dicotyledonous plants were lateral organs, originating 

 below the tip of the embryonic stem or hypocotyl, while 

 the single cotyledon of monocotyledonous plants was 

 considered as a terminal organ. The grass family offers 

 a case in point. The embryo of Indian corn {Zea Mays), 

 for example possesses a well developed cotyledon, called 

 the scutellum; there is little or no trace of a second 

 cotyledon. The embryos of many other grasses, however, 

 possess an organ, the epiblast, homologous in position with 

 the scutellum, and regarded by earlier botanists as a rudi- 

 mentary cotyledon (Fig. 106). Recent studies of Coulter 

 and Land leave little doubt of this as the correct interpre- 

 tation of that organ. 



A study by Bruns (1882) of 82 genera of grasses, repre- 



• According to de Vries (The Mutation theory. 2: 393-456. Chicago, 

 igio) tricotylous intermediate races do not arise by selection but by 

 mutation, tricotyly being the expression of an ancestral character which 

 is latent in the normal species. If the normal character is active and 

 the anomaly semi-latent we have what de Vries calls a ' half-race;" if 

 the normal character becomes latent and the anomaly active, we have a 

 "constant variety." Sometimes an equilibrium is maintained in the ex- 

 pression of the normal character and the anomaly, giving rise to a 

 "middle race," or "eversporting variety." 



