336 



PLANT MORPHOLOGY 



erales (Fig. 285). The dwarf ])ranches, on the other hand, have a large 

 pith, a thin vascular cylinder, and a thick cortex, as in the Cycadales. 

 The leaf traces are double and pass directly into the petiole. Mesarch 

 bundles occur only in the cotyledons, those in all other parts of the plant 

 being collateral and endarch. The tracheids of the secondary xylem have 

 bordered pits in one or two rows. 



Fig. 286. Staminate strobili of Ginkgo biloha. A, clusters of strobili borne on dwarf 

 shoots, two-thirds natural size; B, enlarged view of a dwarf shoot with young leaves and 

 four strobili. 



The strobili of Ginkgo are monosporangiate and dioecious, as in the 

 Cycadales. They are borne at the end of the dwarf shoots, each in the 

 axil of a leaf. 



Staminate Strobilus. The staminate strobili are composed of a central 

 axis bearing many spirally arranged microsporophylls (stamens) forming 

 a loose, catkin-like cluster (Fig. 286). There are no sterile bracts among 

 the sporophylls. The microsporophyll consists of a long stalk ending in a 

 knob that bears two, or occasionally three or four pendent microsporan- 

 gia. The knob is a reduced blade. Ginkgo has continued the "epaulet" 

 type of stamen found among the Cycadofilicales. The microsporangium 

 is eusporangiate in development, the initial being single and hypodermal. 

 The wall consists of four to seven layers of cells, the tapetum being derived 

 from the outermost layer of sporogenous tissue. Dehiscence occurs by 

 means of a longitudinal slit. 



