CONIFERALES 



345 



then divide, so that there are three tiers ; and a division in the lowest 

 tier completes the intra-oval stage of the embryogeny. Some details 

 of these mitoses are shown in Dr. Nilsente Kihldahl's^" study of 

 Pinus (fig. 334). The walls dividing the lower part of the egg into 



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Fig. 334. — Pinus laricio: A, achromatic structures as beginning of third mitosis of 

 fertilized egg; B, late anaphase of third mitosis which will give rise to two tiers of four 

 cells each; C, late telophase of same mitosis with walls nearly complete. The cells of the 

 uppermost tier never have any wall on the side next the general cytoplasm of the egg; 

 X350. — After Dr. Nilsine Kihldahl.-5" 



four tiers are formed on the central spindle in the usual way, while 

 the walls dividing each tier into four cells are formed on the strong 

 threads radiating from the poles of the figure. 



Root, shoot, cotyledons, leaves, and secondary suspensor cells 

 (embryonal tubes) all come from the lowest tier, the other three tiers 

 contributing nothing to these structures. 



