CONIFERALES 347 



Except for the Araucariaceae, the number of free nuclei in the 

 Taxares is regularly higher than in the Pinares. Phyllodadus,^^^ Aiis- 

 trotaxus j"*^^ and Cephalotaxus^'^^ have 8; Podocarpus coriaceus ^^'^^ P. 

 macrophyllus, and P. totora,^^ 16; and Taxus haccata,^^^ 16, with an 

 occasional 32, before walls are formed. Torreya}^^ is exceptional, 

 however, in having only four free nuclei when walls come in. 



The significance of the longer and shorter free nuclear periods will 

 be discussed in the chapter on "Phylogeny." 



In Pinus the four tiers of cells, with four cells in each tier, are al- 

 most geometrical in their symmetry and arrangement; and their 

 function is just as definite. The upper tier, with the distal ends of its 

 cells open to the general mass of the cytoplasm of the egg, is active 

 in the nutrition of the parts below, as long as any food material re- 

 mains in the egg. The next tier below is called the "rosette," from 

 its appearance in vertical view. Its cells are actively meristematic 

 and often develop into embryos. The cells of the next tier elongate 

 immensely, to hundreds of times the length of the original tier, and 

 constitute the suspensor, which may be called the primary suspensor 

 to distinguish it from the structures which appear later and might 

 be mistaken for it. 



Only a part of the cell progeny of the lowest tier develop into em- 

 bryos, and none of the other tiers have any part in the formation of 

 the root, shoot, cotyledons, or leaves. 



A differentiation of the proembryo into tiers is characteristic of 

 conifers, and even in the massive proembryo of the cycads and Gink- 

 go there is a differentiation into regions, although not so sharply 

 marked. 



In the Abietaceae there are, prevailingly, four tiers; in the Taxo- 

 diaceae and Cupressaceae, three tiers; in the Araucariaceae there are 

 three tiers (fig. 336), the upper forming the suspensor, the middle 

 one giving rise to the embryo, and the lower merely forming a tem- 

 porary protective cap, which soon disorganizes. 



In Cephalotaxus three tiers are described : the upper, a rosette; the 

 next below, the suspensor; and the lowest, the group of embryo cells. 

 In the lowest group the lowest cell, or sometimes two cells, acts as a 

 protective cap, as in the Araucariaceae (fig. 337).^°" 



Although the development of the embryo is a continuous process, 



