SEED-BEARING PLANTS 



401 



studied, but possess only two neck cells, and no neck-canal 

 cells. The large egg-cell in the venter is the largest known 

 in the plant kingdom. 



Fig. 298. — Cycas circinalis. Diagram of longitudinal section of a 

 nearly mature seed; 0, outer fleshy layer, with a bundle (0^) of the outer 

 vascular system; s, stony layer of integument; i, inner fleshy layer, with 

 a bundle {i^) of the inner vascular system; c, central vascular bundle. 

 (After Marie C. Stopes.) 



357. Male Gametophyte. — The germination of the 

 microspore and the development of the male gametophyte 

 involve only cell-divisions, but not the growth of new 

 tissue. The mature gametophyte is called a pollen- grain. 

 It consists of three cells: a prothallial cell, a tube-cell, 

 and a generative cell (Fig. 299). There is no structure 

 that can be positively identified as an antheridium, unless 

 the prothalHal cell is considered (as by some), as repre- 

 senting the antheridium. The pollen-grain has two 

 coats — an outer and an inner. 



358. Pollination. — When the pollen is mature it is 

 scattered by the wind, and some of the grains lodge, by 



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