352 PLANT MORPHOLOGY 



Following the establishment of the embryonal, suspensor, and rosette 

 tiers, the four cells of the lowest tier divide and interpose another tier 

 between them and the primary suspensor cells, thus forming secondary 

 suspensor cells (embryonal tubes) . This behavior may be repeated once 

 or twice again. The primary suspensor cells elongate and thrust the 

 lower cells into the female gametophyte tissue. The four rows of lower 

 cells now separate and the secondary suspensor cells elongate. Elonga- 

 tion of the suspensors, both primary and secondary, continues to such an 

 extent that they become coiled and twisted (Fig. 300//). 



Each of the four terminal cells gives rise to a separate embryo. Mean- 

 while one or more cells of the rosette tier may divide to form a rosette 

 embryo. These ordinarily do not develop very far, however, before they 

 disintegrate. As a result of receiving unequal amounts of food, the four 

 primary embryos grow at different rates. The largest finally survives and 

 the others become aborted. Thus the mature seed, with rare exceptions, 

 has only one embryo. 



In the Abietaceae, Taxodiaceae, and Cupressaceae four free nuclei are 

 formed in the egg before walls appear. In the other families a larger num- 

 ber of nuclei are produced — as many as 32 or 64 in the Araucariaceae. 

 After the appearance of walls, the proembryo, as a rule, consists of four 

 tiers of cells in the Abietaceae, but of only three tiers in the Taxodiaceae, 

 Cupressaceae, and Araucariaceae. In many conifers the cells of the 

 lowest tier of the proembryo do not separate to form four embryos, as 

 they do in Pinus, but remain together to form a single embryo, while the 

 cells of the rosette tier collapse instead of giving rise to rosette embryos. 

 In these conifers several embryos usually begin to develop in the same 

 ovule, but each comes from a different fertilized egg. 



Thus polyembryony in the Coniferales is of two types: (1) cleavage 

 polyembryony, where multiple embryos arise from the splitting of a single 

 embryo; and (2) simple polyembryony, where more than one fertilized egg 

 in the same ovule gives rise to an embryo. Each type is characteristic of 

 particular genera. In the Abietaceae, for example, cleavage polyembry- 

 ony is a feature of Pinus, Cedrus, and Tsuga, while simple polyembryony 

 is characteristic of Larix, Picea, Pseudotsuga, and Abies. In both types of 

 polyembryony only one embryo in each seed reaches maturity, the others 

 degenerating. 



When mature, the conifer embryo consists of a hypocotyl terminating 

 in a radicle at the suspensor end and a minute plumule at the opposite end, 

 the plumule being surrounded by two or more cotyledons (Fig. 301) . The 

 number of cotyledons ranges from 2 to 18, but more conifers are dicotyle- 

 donous than polycotyledonous. The Abietaceae show the largest number 

 and the greatest variability. The ripe seed of conifers is generally brown. 

 The outer and inner fleshy layers of the integument become very thin, the 



