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PLANT MORPHOLOGY 



where they meet the pollen tubes. Then an egg nucleus fuses with one of 

 the male nuclei, after which it passes back into the female gametophyte. 

 The cells in the lower region, after becoming uninucleate, continue to 

 multiply, even after fertilization has taken place. They form a nutritive 

 tissue. 



Fig. 305. Welwitschia mirabilis. A, young staminate strobili, natural size; B, ovulate 

 strobilus, natural size; C, staminate "flower" with bracts removed, showing the six stamens 

 united below and the sterile ovule with a long twisted micropylar tube, X8; D, longi- 

 tudinal section of ovule, showing inner integument forming micropylar tube and outer 

 integument forming a wing, X3. {A, B, C, after Hooker; D, after Church.) 



In Gnetum the female gametophyte begins its development with free- 

 nuclear division. However, there is no wall formation except, in some 

 species, in the basal region, where a small-celled nutritive tissue is formed. 

 Each nucleus in the micropylar region is a potential egg nucleus, several 

 usually becoming organized as eggs but only one being fertilized. After 

 fertilization, the gametophyte becomes cellular throughout. At first the 

 cells are multinucleate but later become uninucleate, as in Welwitschia. 



Male Gametophyte. In Ephedra the microspore cuts off two prothal- 

 lial nuclei but only the first is organized as a cell (Fig. 307). These are 



