GINKGO. 251 



integument, which in the ripe seed forms a thick fleshy covering- 

 surrounding a hard woody shell ; the nueellus possesses a well- 

 marked pollen-chamber, and in the mature ovule the greater part 

 of the nucellar tissue is reduced to a thin papery layer enclosing 

 a large embryo- sac which usually contains two archegonia. After 

 fertilization, which may occur either before or after the ovule has 

 fallen from tbe tree, the egg-cell develops directly into an embryo 

 with two cotyledons. 



The secondary wood of Ginkgo is composed of traeheids with 

 numerous bordered pits on the radial and not uncommonly on the 

 tangential walls. Resin ducts occur in abundance, both in the pith 

 and in the cortical tissues. 



It is unnecessary to enter here into a detailed examination of 

 the histological characteristics of Ginkgo ; but we may briefly 

 summarize some of the features in which the genus agrees with 

 the Cycadaceae. 



1. The archegonia possess two neck-cell-. 



2. The seeds are situated at the margin of the carpellary leaves, 



and have a fleshy testa. 



3. The male reproductive cells have the form of motile 



spermatozoids. 



4. The short shoots bear scale-leaves and foliage-leaves, and 



present a close resemblance to the stems of Cycads. 



5. The presence of a large pollen-chamber at the apex of. the 



nueellus. 



6. The existence of centripetal wood in the cotyledon-stalk, as 



also in the foliage-leaf, petiole, and elsewhere. 



7. Resemblances in the anatomy of the short shoots to features 



characteristic of Cycadean structure. 



AVe need not concern ourselves for the present with the various 

 views that have been held as to the morphology of the female 

 flowers, a subject which has given rise to no little controversy, nor 

 need we attempt an exhaustive description of the various features 

 of morphological and phylogenetic importance exhibited by both 

 vegetative and reproductive organs. 1 



The leaves of Ginkgo biloba, which Kamipfer and others have 

 compared with those of the maidenhair fern, are of special 



1 Vide Seward & Gowan (00). 



