GINKGOALES 



203 



supports the interpretation that the peduncle is a stem, bearing two 

 sporophylls, the collars, each bearing an ovule (fig. 218). When a 

 peduncle bears three ovules, there are six bundles; and when there 

 are several ovules, the peduncle has double that number of bundles, 

 so that a transverse section has a zone of wood like that in a stem. 

 In general, the development of the ovule resembles that of a cy- 

 cad (figs. 219 and 220). There is a prominent nucellus with a large 

 pollen chamber, surmounted by a beak which becomes brown and 

 hard. The single integument, with the lower part of the ovule, be- 

 comes differentiated into three layers, the outer fleshy layer, and 

 inner fleshy layer, with a stony layer between them. The develop- 



FiG. 218. — Ginkgo biloha: A, transverse section of petiole of leaf, showing two vascu- 

 lar bundles; B, similar section of peduncle, showing four bundles; X17. 



ment of the inner fleshy layer differs from that of a cycad, for its cells 

 are very thin walled and watery until the ovule reaches a considerable 

 size. It then develops as in a cycad, finally becoming a thin, dry, 

 papery brownish membrane. The outer fleshy layer remains fresh, 

 greenish, and juicy until the frost comes in autumn. The ovules then 

 fall to the ground, the fleshy layer becomes wrinkled, but still re- 

 mains watery, and has a characteristic odor which is unpleasant to 

 most people. The ovules are poisonous to some people, causing sores 

 on the hands or other parts which may be touched. 



The vascular supply of the ovule is very scanty. Two strands 

 enter the inner fleshy layer, and, without any branching, extend to 

 the free part of the nucellus. The entire outer vascular system, so 

 prominent in the outer fleshy layer of the cycad ovule, is entirely 

 lacking. In the rare cases in which the ovule is triangular in trans- 

 verse section, there are three bundles in the inner fleshy layer. 



