GINKGO ALES 



191 



although only one cell wide, range from one to sixteen cells in height, 

 with rays three to six cells high very common (figs. 204 and 205). 

 The rays remain alive for a long time, retaining their nuclei, proto- 

 plasm, and starch for 30 years or 

 more. 



With the exception of the medul- 

 lary rays, there are no parenchyma 

 cells, like the thin-walled cells of the 

 cycads, interspersed with the woody 

 tracheids. 



The leaf. — ^The beautiful leaf, with 

 its symmetrical dichotomous vena- 

 tion, has given Ginkgo its colloquial 

 name, the Maidenhair Tree, because 

 the leaves on the spur shoots resem- 

 ble those of Adiantum, the Maiden- 

 hair Fern. The leaves on the long 

 shoots are mostly bilobed, the fea- 

 ture which suggested the specific 

 name; but the leaves on the spurs 

 usually have only a wavy margin, 

 with none of the deep lobing. Leaves 

 at the top of the tree, on first-year 

 long shoots, and especially the leaves 

 of seedlings, are very deeply lobed; 

 and besides the two deep primary 



lobes may have two or three secondary lobes on each side, so that 

 they approach the deeply and narrowly lobed leaves of the extinct 

 Baiera (figs. 206, 207). 



With so much variation in the leaves, species based upon leaf 

 characters alone are open to more or less suspicion. 



The dichotomous venation of the leaf is very regular. In the 

 petiole there are two strands, each, by repeated forking, forming the 

 venation of its side of the leaf (Fig. 208). It may be that the more 

 vigorous growth of seedlings and long shoots may be responsible for 

 the bilobed character of their leaves, while the leaves of the slow 

 growing spurs are seldom bilobed. 



Fig. 203. — Ginkgo biloba: longi- 

 tudinal section of mature wood show- 

 ing pitting and "Bars of Sanio." — 

 After Jeffrey.^"! 



