CONIFERALES (PINACEAE) 295 



the transition from the four-nucleate to the eight-nucleate stage, as in 

 most Pinaceae. Only one of the cells of the proembryo develojjs the 

 embryo, and the one next above forms the long suspensor. while the 

 other proembryonal cells soon disappear. 



In these three tribes of Pinaceae, therefore, the proembryo con- 

 sists of twelve to sixteen cells, arranged in three (Taxodineae and 

 Cupressineae) or four (Abietineae) tiers, the fewer cells belonging 

 to the Cupressineae; a statement that must exclude Sequoia, Wid- 

 dringtonia, and Callitris. Unfortunately, the proembryo of none of 

 the Araucarineae has been investigated. 



THE SUSPENSOR AND EMBRYO 



The suspensor cells elongate enormously, thrusting the embryo 

 cells into the endosperm, and usually becoming very long and tortu- 

 ous (figs. 364, 365). In such cases as Pinus, in which there is a sus- 

 pensor cell for each of the four embryo cells, four separate embryos 

 may begin to develop, which is the usual method, or all four cells 

 may form a single embryo. E\-en in such cases as Taxodium, in 

 which there is inequality in the number of suspensor and embryo 

 cells, the former may or may not separate from each other, resulting 

 in one or more embryos. Of course in such a case as Thuja, in 

 which the embryo cells are not in one plane, and one of them is terminal, 

 a single embryo is formed. Even a single embryo cell may give rise 

 to two embryos, if the first division is vertical and the two daughter 

 cells become organically dissociated. The indifference of these cells 

 in reference to embryo-formation is very marked, an embryo being 

 formed by a group of proembryonic cells (the "embryo tier"), by 

 any member of this group, or even by its daughter cell. 



In many of the Pinaceae "embryonal tubes," haustorial in func- 

 tion, are developed from the basal cells of the embryo, and are found 

 investing the end of the suspensor and the base of the embryo, some- 

 times being exceedingly numerous and conspicuous (figs. 365, 366). 



There is no definite sequence in the early divisions of embryo- 

 formation. Transverse or vertical walls may occur at any division, 

 although in the case of an embryo developing from a single cell the 

 first two or three divisions are generally transverse. The organization 

 of the great body regions has not been investigated, so that between 



