648 MINNESOTA BOTANICAL STUDIES. 



strand of somewhat smaller cells running from the plumule di- 

 rectly through to the base of the embryo (Figs. 12-18). From 

 this main axis, at a point just above its middle (Fig. 18), lateral 

 strands radiate outward and traverse the lobes of the cotyledon. 

 This stage culminates in an embryo as represented in Fig. 9. 



Stage D. Mature embryo. This stage which is reached with 

 the completion of intraseminal growth may be discussed under 

 the following captions : 



1. Maturation of the Plumule. The first foliage leaf arises 

 on the axis of the plumule morphologically opposite the cotyle- 

 don (3, Fig. 14) (/. ., on the front side of the plumule axis). 

 The second foliage leaf arises opposite the first (c, Fig. 19). 

 The stipules of the first and second leaves grow over and 

 around the apex of the stem and enclose the next two leaves, 

 which are also preformed in the seed (Fig. 23). The lysige- 

 nous cavities of the stem and leaf petioles very early appear, 

 as seen in Figs. 19 and 22. The structure of the mature plu- 

 mule has been many times described and needs no further treat- 

 ment here. Its development, however, may be well under- 

 stood by a comparison of Figs. 10, 14, 17, 19, 22 and 23. 



2. Origin and Maturation of the Radicle. The radicle 

 (r, Fig. 22) originates opposite the insertion of the plumule 

 and is but a vestigial structure not developing into a functional 

 primary root upon the germination of the seed. It only be- 

 comes apparent at a late stage in the development of the em- 

 bryo and is usually completely enclosed by an outgrowth of the 

 cotyledonary tissue. Fig. 24 shows a longitudinal section of 

 the hypocotyl of a seedling which had already developed sec- 

 ondary roots from the epicotyl. 



3. Maturation of the Cotyledon. The edges of the cotyledon- 

 ary lobes soon meet through tangential growth, thus forming a 

 tube in which the plumule stands surrounded by the endosperm. 

 The cells directly beneath the epidermis near the apex of the 

 lobes remain densely protoplasmic and differentiate into a dis- 

 tinct palisade tissue (Figs. 20, 21). This undoubtedly forms a 

 nursing area, being in contact with the lower portion of the 

 nucellus which persists for some time and is still evident in the 

 seed as a thicker, more distinct portion of the testa (a, Fig. n). 

 The lobes through more rapid elongation soon reach their 

 maximum length and then by somewhat slower tangential and 

 radial growth the embryo acquires its ultimate spherical form. 



