1915] 



COULTER ORIGIN OF MONOCOTYLEDONY 



179 



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embryo, continuing beyond the cotyledonary ring or sheath, 

 and producing a variable number of leaves. 



The early appearance and rapid develop- 

 ment of these leaves seems to account for 

 the abortion of one of the growing points. 

 I am convinced that if grass embryos had 

 been the only monocotyledonous embryos 

 studied, we should never have heard of 



terminal cotyledons. 

 Some common grasses, whose embryos 



have been figured by Bruns, 1 may be used 



to illustrate stages in the abortion of the 



second cotyledon. The abortion always is 



accompanied by the diver- 

 sion of the growth of the 

 whole cotyledonary zone in connection with 

 the growing point that remains active; so 

 that growing tissue is not suppressed, but 

 develops as one structure rather than as two. 



In Zizania aquatica (fig. 

 2), the so-called epiblast is 

 very conspicuous, arising 



Fig. 5. Embryo 

 of 8 par Una cyno- 

 suroides: s, scutel- 

 lum; e, epiblast; c, 

 coleoptile; X 13. — 

 After Bruns. 



Fig. 6. Embryo 

 of Leptochloa arab- 

 ica : s, scutellum ; 

 e, epiblast; o, cole- 

 optile; X 44. — 

 After Bruns. 



as distinctly from the per- 

 ipheral cotyledonary ring as does the so- 

 called scutellum, and attaining at least one- 

 quarter to one-third of its length. This 



unusual development of the second cotyledon 

 is associated with the fact that the stem 

 axis above the cotyledons develops a long 

 internode, so that the first leaves begin to 

 appear at an unusual distance from the 

 origin of the cotyledons. In fact, in this 

 case the length of the second cotyledon is 

 approximately the length of the first inter- 

 node, and where the leaves begin this coty- 

 ledon ends. 



In Leersia clandestine/, (fig. 3), the second cotyledon (epi- 

 blast) approaches the large cotyledon in length even more 



1 Bruns, Erich, Der Grasembryo. Flora 76: 1-33. pi. 1-2. 1892. 



Fig. 7. Embryo of 

 Triticum vulgare: s, 

 scutellum ; e, epi- 

 blast ; c, coleoptile ; 



X 22.— After Bruns. 



