1315] 



COULTER — ORIGIN OF MONOCOTYLEDONY 



177 



the embryo of grasses, with all of its unusual structures 

 would be interpreted in terms of a rigid conception 



of the 



monocotyled 



embry 



in other words 



that the conventional monocotyledonous em- 

 bryo would be read into the grass embryo. 

 There is no better illustration of the com- 

 pelling power of a preconception than this 

 treatment of the grass embryos, for it so 

 happens that they show all the intermedi- 

 ate stages between dicotyledony and mono- 



cotyledony. 

 Very early in the history 



of this subject, the scutellum 



came to be recognized 



a 



5 



tyled 



The corollary 



this propositi 



however 



c— 



was that it must be recog- 

 nized also as a terminal 

 structure. Any one who has 

 seen the vascular system of 

 the embryo of corn (fig. 1), 

 the most highly specialized 

 of all grass embryos, with its 

 distinct axial cylinder, made 

 up of stem cylinder and 

 hypocotyl cylinder, and the 



er— 



tyle d 



strands lead 



off from the intermediate 



tyled 



plat 



just as 



do the strands of anv lateral 



tyled 



will understand 



Fig. 2. Embryo 

 of Zizania aquat- 

 ica : s, scutellum ; 

 e, epiblast; c, cole- 



optile; X 11. 

 After Bruns. 



the great difficulties in the 

 way of interpreting this 

 cotyledon as a terminal 

 structure. 

 The structure which pre- 



Fig. 1. Embryo 

 of Zea Mays: s, 

 scutellum; c, cole- 



optile ; the vascu- 

 lar cylinder of the 

 embryo is shown, 

 made up of stem 

 cylinder and hypo- 

 cotyl cylinder, also 

 the lateral origin 

 of the cotyledon 

 (scutellum) from 

 the cotyledonary 

 vascular plate; op- 

 posite the vascular 

 connection o f the 

 cotyledon there ap- 

 pears a group of 

 procambium cells, 

 marking the origin 

 of another cotyle- 

 donary strand con- 

 nected with the 

 suppressed second 



cotyledon (epi- 

 blast) ; X18. 



sented the greatest difficulty, however, was the epiblast, usually 

 defined as a small scale "opposite" or "over against" the 



