EXPLANATION OF PLATE XVIII. 
Diagrammatic longitudinal sections (tangential to axis of the spike) through^the 
Gnetalean female flowers and the ovule and ovular envelope of the male flower 
of Welwitschia. Only the lower part of the micropylar tube is shown, except in 
Welwitschia <J, in which its middle portion is omitted. 
I, II, III = envelopes in centrifugal order (strong lines = vascular" system). 
A = base of androecial tube in Welwitschia $ . 
E.S.= embryo sac. 
N=nucellus. 
S = " stigma " of Welwitschia $ , 
