192 



GYMNOSPERMS 



I'he veins occasionally have a few centripetal tracheids, and thus 

 have an indistinct mesarch structure. A single strand enters the 

 cotyledons, distinctly mesarch below, but becoming exarch higher 

 up. 



Fio. 204 



Fig. 20s 



Fig. 204. — Ginkgo biloba: longitudinal tangential section of long shoot, showing med- 

 ullary rays, one to three cells in longitudinal extent; X180. 



Fig. 205. — Ginkgo biloba: longitudinal tangential section spur shoot, showing medul- 

 lary rays, one to fifteen cells in longitudinal extent; X 180. 



The mucilage cavities in the leaves are elongated, i-5mm. in 

 length, and are nearly as conspicuous as the veins themselves. They 

 take the same direction as the veins, and are usually spaced about 

 half-way between adjacent veins. Many cells contain a large crystal 

 of calcium oxalate, and tannin cells are abundant. 



The two strands in the petiole are endarch, with spiral protoxylem 

 elements. At a distance of several cells from the protophloem there 

 is a sheath of thick-walled cells, and, just within it on the protoxylem 

 side, there are often some thick-walled suberized tracheids (fig. 209). 



I 



