Lyon: THE EMBRYOGENY OF GINKGO. 281 



large member. This embryo possessed the stele of a typical 

 tricotyledon. In several similar cases noted, the primordia had 

 assumed independent growth after a period of conjoint develop- 

 ment, so that the cotyledons were not united throughout their 

 entire length (fig. jj). 



Five leaf-primordia are usually to be distinguished on the 

 plumule of a mature embryo. In a dicotyledonous embryo, 

 the cotyledons are often not so truly diametrically opposite each 

 other, as are the two following leaves ; one of the latter being 

 regularly larger than the other, and occupying more space 

 between the cotyledons (Jigs. 75-77). Beyond the cotyledons 

 and first pair of plumular leaves, a decussate arrangement does 

 not obtain in the embryo (fig. //). In a tricotyledonous em- 

 bryo, the leaves are arranged spirally and have an indeterminate 

 divergence (fig. /p). 



In a mature seed the embryo extends through one half to 

 two thirds the length of the gametophyte. The cotyledons are 

 of equal length ; often slightly reflexed at their tips (figs. 24.- 

 2j), and when the embryo is removed from the seed, they 

 usually separate of themselves. Frequently, a considerable 

 mass of disorganized gametophytic tissue is included between 

 the cotyledons. The stem and root regions and also the prox- 

 imal halves of the cotyledons are white ; the ends of the coty- 

 ledons being yellowish or light green in color. The resin- 

 reservoirs are most numerous in the stem and basal portions of 

 the cotyledons, and in fresh embryos stand out as prominent 

 greenish or amber-colored pustules. 



When an embryo is placed in a chromic acid fixing solution, 

 the root-cap region turns brown. A section (figs. 7, 77, 12) 

 shows this color-change to be limited to the peripheral layer of 

 cells; the change being due to a change in their contents. The 

 fixing-fluid has a similar effect on the contents of the secretory 

 vessels, and also darkens the resin-globules. 



B. The Seedling. 



i. Morphogenesis. When seeds are placed under favorable 

 conditions for germinating, the hard shell is cracked, at the 

 micropylar end, by the swelling of the gametophyte. Through 

 this opening the body of the embryo is thrust out by the elon- 

 gation of the cotyledons. The root immediately turns down 

 into the soil (fig. jp), and as soon as the stem is free from the 



