SPERMATOPHYTA 349 



The young ovulate cone is ready for pollination soon after emerging 

 from the l^ud (Fig. 291). Its scales separate slightly and a pollination 

 droplet exudes through the micropyle of each ovule. This droplet 

 catches some of the pollen grains and, upon evaporating, draws them 

 down into contact with the nucellus. Following pollination, the scales 

 close and the cone begins a long period of growth. The pollen grains soon 

 germinate. The apical end of each pollen tube grows downward into the 

 nucellus, not laterally as in the cycads. 



The tube nucleus moves into the pollen tube, while the generative cell, 

 remaining at its basal end, soon gives rise to the stalk cell and body cell 

 (Fig. 298£'). These pass into the tube and considerably later the body 

 cell divides to form two male cells that are usually equal in size and are 

 always nonciliated (Fig. 299A). In the Abietaceae the two male nuclei 

 are surrounded by cytoplasm derived from the body cell but are without 

 a cell wall. They remain inside the body cell until just before the time 

 of fertilization. 



The pollen tube comes in close contact with the archegonium and the 

 tip ruptures, discharging its contents into the egg. One of the male nuclei 

 approaches the egg nucleus and the tw^o come together (Fig. 2995). If 

 the second male nucleus also enters the egg, it soon disintegrates. The 

 cytoplasm surrounding the male nucleus mingles with the egg cyto- 

 plasm. In conifers with highly organized male cells, after entering the 

 egg, the protoplast escapes from the cell wall and the cytoplasm remains 

 in contact with the male nucleus, finally forming a conspicuous sheath 

 around the fusing nuclei. The male and female nuclei do not fuse in the 

 resting condition, but each forms a group of chromosomes that become 

 arranged on a common spindle (Fig. 299C, D). Completion of the mitosis 

 gives rise to the first two nuclei of the proembryo. 



In Pinus and Juniperus the interval between pollination and fertiliza- 

 tion is slightly more than a year, but in most other conifers it is less, some- 

 times only a month or two. During this time the development of the 

 female gametophyte and the growth of the pollen tube take place. 



Embryo. In Pinus four free nuclei are formed within the fertilized 

 egg as a result of two successive mitotic divisions (Fig. 300A, B). These 

 nuclei move to the base of the egg, where they become arranged in a hori- 

 zontal plane. Each nucleus divides and walls come in, forming two tiers 

 of four cells each (Fig. 300C', D). The cells of the upper tier, which 

 remain open above, divide again and then the cells of the lower tier divide. 

 The proembryo now consists of four tiers of cells with four cells in each 

 tier (Fig. 300£', F). The lowest tier gives rise to four embryos; the next 

 tier forms the primary suspensor cells; the next one constitutes the 

 "rosette tier," which may later give rise to four embryos also; while the 

 four upper cells are part of the general cytoplasm of the egg, which serves 

 as a large food reservoir. The nuclei of the upper cells soon disintegrate. 



