178 
WALTER P. THOMPSON 
from the argument that the points just mentioned preclude any 
possibility of the Bennetitalean ancestry of Angiosperms, the great 
resemblance between the latter group and the Gnetales are not satisfied 
by any such theory. Moreover the evidence relates the Gnetales, 
not to the Bennetitales but to the Conifers. 
It remains to remark that the Amentales are the Angiosperms 
which most closely resemble the Gnetales and appear therefore to be 
the most primitive order of the phylum. There is the possibihty that 
both Gnetales and Amentales are somewhat reduced as is indicated by 
the flower structure of the former. 
13. Summary 
I. Actual Conditions. — Abnormal strobili are found in which the 
flowers are arranged in a spiral, the whole resembling very much a 
catkin of the Amentales. Vessels of the Ephedra type are present 
in the axis of the strobilus. 
The development of the microsporangium takes place in the usual 
way. Two layers of parietal cells are formed. There is no endo- 
thecium. The tapetum is developed from sporogenous cells. The 
period of development is very short. 
In the development of the megasporangium the three envelopes 
arise in acropetal succession. The style develops conducting tissue. 
Two to four mother cells are formed, two or three produce megaspores, 
and two or three megaspores develop into embryo-sacs. 
The usually abortive ovules of the staminate strobili frequently 
possess embryo-sacs and may produce seeds. 
In the male gametophyte no prothallial cells are formed; the 
microspores frequently germinate in the style at a distance from the 
nucellus; the stalk cell never passes into the pollen tube; distinct 
male cells are formed while the tube is on the nucellus. 
The female gametophyte develops in typical Gymnospermic 
fashion until approximately 256 free nuclei are present in G. gnemon 
and 512 in other species. No cell walls are formed before the entrance 
of the pollen tube. Definite eggs are organized. Two or three 
gametophytes usually develop in each ovule. 
After the entrance of the pollen tube rapid divisions occur in the 
female gametophyte. Multinucleate compartments are then formed 
and all the nuclei in each compartment fuse. The further growth 
of the endosperm occurs by the division of these compartments. In 
G. gnemon only a few fusion nuclei participate in endosperm formation. 
