SPERMATOPHYTES 



217 



Fig. 4S0. — Male gametophyte 

 of Dacrydium (one of the podo- 

 carps) at shedding stage of pollen, 

 showing foiir vegetative cells, stalk 

 and body cells (formed by the 

 generative cell), and the tube 

 nucleus. — After Miss Young. 



there is much nucellar tissue to be traversed before the female gameto- 

 phyte with its archegonia is reached. For the first time, the pollen 

 tube acts as a carrier of the male cells, the 

 body cell, which is to divide, entering the 

 tube and remaining near its tip as it ad- 

 vances through the tip of the nucellus (fig. 

 479). The tube may advance directly 

 towards the archegonia or it may pursue a 

 devious route, in some cases not reaching 

 the archegonia until during the second 

 season. When an archegonium is reached, 

 its neck is broken through and the contents 

 of the tip of the pollen tube are discharged 

 into the egg (fig. 481). 



Embryo. — In the development of the 

 embryo from the fertilized egg, there is a 

 variable amount of free nuclear division, 



from four free nuclei {Torreya, fig. 482) up to thirty-two, in strong 

 contrast with the very numerous free nuclei appearing in the embry- 



ogeny of cycads and of 

 Ginkgo. After the free 

 nuclei are formed, walls 

 appear, and the resulting 

 tissue (proembryo) fills the 

 egg. In general, there are 

 about three tiers of cells in 

 the proembryo, the lowest 

 (innermost) one usually con- 

 sisting of a single cell, so 

 that the proembryo has a 

 general conical outline (fig. 

 483) . An elongation of cells 

 begins in the upper (outer- 

 most) tier (fig. 484), and 

 this is continued by the 

 middle tier, so that the 

 terminal cell, which is to 

 form the embryo, is thrust 

 out of the archegonium and 



Fig. 481. — Fertilization of Torreya: the pollen 

 tube has discharged its contents into the egg, and 

 the male and female nuclei are fusing. — After 

 Coulter and Land. 



