GIlfKGO. 251 



integumeat, whicli in the ripe seed forms a thick fleshy covering^ 

 surrounding a hard Tvoody shell ; the nucellus possesses a well- 

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

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

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

 fertilization, which maj- occur either before or after the ovule has 

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

 with two cotyledons. 



The secondary wood of Ginkgo is composed of tracheids with 

 numerous bordered pits on the radial and not uncommonly on the 

 tangential walls. Kesin 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 briefl}" 

 summarize some of the features in which the genus agrees with 

 the Cycadaceae. 



1. The archegonia possess two neck-cells. 



2. The seeds are situated at the margin of the carpellarj- 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 



nucellus. 



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



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



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



characteristic of Cyoadean structure. 



We 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.^ 



The leaves of Ginkgo hiloha, which Krempfer and others have 

 compared with those of the maidenhair fern, are of special 



' Vide Seward & Gowan (00). 



