20 MINNESOTA BOTANICAL STUDIES. 



ylar end of the embryo-sac. As a result of this change the embryo 

 comes to occupy the bottom of the cup-Hke depression which forms 

 the middle of the invaginated part of the seed. The growth of the 

 embryo continues with Httle change of form until it is about 500 mic. 

 long, and 200 mic. in diameter at the widest part. At about this time 

 growth in length in the distal part of the embryo becomes confined 

 largely to the peripheral region, while growth in thickness continues 

 throughout. The result is that the distal end of the embryo be- 

 comes flattened and apparently in some cases even slightly concave. 

 At about this stage the primordia of the cotyledons appear, as two 

 mound-like outgrowths occupying opposite sides of the margin of 

 the distal end of the embryo. They grow rapidly in length, while 

 the margin of the embryo between them grows little or none. The 

 morphological apex of the embryo which eventually gives rise to 

 the growing point of the stem is at this stage very little or not 

 at all prominent. The shape of the embr^'o at the time of the ap- 

 pearance of the cotyledons is usually nearly or quite symmetrical, 

 but a slight asymmetry occurs in some cases, one side of the embryo 

 being very slightly higher and more prominent than the other, or 

 the cotyledonary mounds developing at points, not quite diametric- 

 ally opposite. This asymmetry when present appears to afifect the 

 pear-shaped embryo quite independently of the orientation of the 

 cotyledons. In no case do the cotyledons appear to arise first as 

 a single crescentic ridge, but always as distinct mound-like out- 

 growths which are usually very symmetrically placed on opposite 

 sides of the distal end of the pear-shaped embryo (PI. X; Fig. 12, 

 PI. V). 



The cotyledonary mounds grow rapidly into somewhat broad, 

 thick, blunt, and usually very symmetrical cotyledons. While it 

 appears possible that the sinus between the cotyledons may be some- 

 times a little deeper on one side than on the other ownng to the 

 possible slight asymmetry of the embryo, yet this is certainly not 

 the rule, and it is probable that in almost all cases in which the trans- 

 verse sections seem to indicate this peculiarity, the appearance is 

 due to slightly oblique sectioning.^ 



1 This conclusion is diametrically opposed to that reached by Mr. C. E.. 

 Lewis (Studies on some Anomalous Dicotyledonous Plants, Bot. Gaz. 37: 127, 

 1904.) In studying the origin of the cotyledons sections were made of a large 

 number of embryos in which the cotyledonary primordia were 20 mic. — 40 mic, 

 in height (the cells of this part of the embrjo are about 20 mic. in diameter) 



