756 A TEXTBOOK OF THEORETICAL BOTANY 



of which divides to form two male nuclei. Lastly there is a spore-cell nucleus, 

 which later becomes the tube nucleus (Fig. 758). This differentiation is 

 completed before the shedding of the pollen, and the disappearance of the 

 two prothallial cells follows almost immediately. 



On arrival at the ovule, germination of the pollen grains is very rapid. 

 The extine is thrown oft" and a germination tube appears at once, into which 

 pass the tube nucleus and the two male nuclei, accompanied by the nucleus 

 of the stalk cell. 



Fertilization occurs less than twelve hours from pollination, a remarkable 

 contrast to the prolonged delay characteristic of most Gymnosperms. The 

 pollen tube penetrates between the neck cells of the archegonium and the 

 two male nuclei pass directly into the oosphere. 



The Female Gametophyte. 



The thin inner integument grows out into a long pollen-catching micro- 

 pylar tube, which becomes twice as long as the perianth. The apex of the 

 nucellus develops a pollen chamber, the bottom of which goes right through 

 to the apex of the prothallus (Fig. 759). The pollen thus actually reaches 

 the female prothallus itself, a feature in which this type of ovule is unique. 

 The prothallus develops from the lowest of a row of four megaspores. There 

 is first a period of free nuclear division, which reaches at least the 256-nuclear 

 stage before cell walls are formed, but the details are not known. The mature 

 prothallus is elongate and has two clearly distinguishable regions. The 

 micropylar region, usually with two archegonia, is formed of elongated, 

 thin-walled cells, with a central strand of almost empty cells running down- 

 wards to the lower, antipodal, region of the prothallus. The latter region is 

 composed of small, regular cells with much denser contents, especially in 

 the central cells, which are evidently a nutritive tissue. Two outer layers, 

 called the haustorial layers, apparently act as absorbers of material from 

 the nucellus. The archegonia are also elongated and have a jacket layer of 

 cells which is not very well defined. The archegonia develop from superficial 

 cells of the prothallus and have at first a neck with only three tiers of cells, 

 but, by the upgrowth of the prothallus apex, the number of neck cells becomes 

 so great that eight or more tiers of cells may overlie the top of the oosphere, 

 which is thus more deeply sunken in the prothallus than in any other 

 living plant. This upgrowth of the prothallus should be noted in 

 comparison with the formation of much more pronounced upgrowths in 

 Welwitschia. 



In the oosphere there is a large nucleus, with a dense plasmatic sphere 

 around it, and a large ventral canal nucleus but no ventral canal cell. 



Fertilization. 



Both of the male nuclei are released into the archegonium, but only the 

 first unites with the female nucleus. The second male nucleus and the 



