282 MINNESOTA BOTANICAL STUDIES. 



gametophyte it begins to grow in the direction opposite that fol- 

 lowed by the root {fig- •//)• With the elongation of the stem, 

 the plumular leaves are quickly displaced from their original 

 relative positions {figs. 42-47). The first two or three leaves 

 directly following the cotyledons do not develop the character- 

 istic blades, but remain small and scale-like {fig. 47). The stem 

 soon stops its rapid elongation, and having expanded a rather 

 close crown of leaves at its apex {figs. 47, 48), spends the 

 remainder of the first season in strengthening and protecting 

 itself by secondary growth in its tissues. 



As is the rule among gymnosperms, the primary root of 

 Ginkgo persists as the tap-root. During the first season it 

 develops numerous short secondary roots {figs. 47, 48). 



Upon the germination of the seed, the proximal portions of 

 the cotyledons, which, as we have already noted, are well sup- 

 plied with resin-reservoirs, protrude from the gametophyte as 

 large, arching petioles {figs. 41-44). The portion of the coty- 

 ledon, remaining within the gametophyte, enlarges considerable, 

 {fig. 48), its greatest diameter being about twice that of a coty- 

 ledon of a mature embryo of the seed. The cotyledons persist 

 throughout the first season. Fig. 48 is a photograph of a liv- 

 ing seedling at the end of its first summer's growth. It was 

 taken in October, after the seedling had experienced several 

 quite heavy frosts. The remains of the seed were removed 

 exposing the large fleshy cotyledons, which sprang apart of 

 themselves as soon as freed from the tissue of the gametophvte. 



A large terminal bud is the only prominent one on the stem 

 during its first winter; and, if uninjured, is the only one to 

 become active during the second season. Fig. 49 is a photo- 

 graph of a young plant which has made a considerable portion 

 of its second year's growth. The leaves all possess fully 

 expanded blades, and are well distributed along the stem. 

 During its second season the plant develops a much-branched 

 root-system. 



2. Histogenesis. — As has already been mentioned in discuss- 

 ing the embryo of the seed, the first permanent vascular tissue 

 is differentiated in the cotyledons. As the embryo begins its 

 extraseminal development, differentiation progresses from the 

 cotyledons towards the root- apex. The two bundles of a coty- 

 ledon unite eventually to form one of the persistent bundles 

 which extends throughout the length of the root. If the seed- 



