THE GYMNOSPERMS 



369 



533. Zamia. Zamia is also 

 dioecious, and the male and female 

 plants both bear cones. In the 

 staminate cones (fig. 366) the spore- 

 bearing leaves are short and stout, 



Fig. 367. 

 Pistillate cone of Zamia. 



Fig. 368. 



Zamia, one scale of the pistillate cone, showing 

 the two ovules. 



somewhat triangular wedge-shaped, and very closely crowded. 

 Upon the underside are numerous small oval spore cases similar 

 to those of cycas and some of the lower ferns, which contain 

 numerous small spores. The pistillate or carpellate cone is 

 similar in external appearance but larger. The spore-bearing 

 leaves are similar in form but bear two large spore cases, one 

 on either side but covered by the broadened outer end. 



534. Life history of the cycads. The life history of the 

 cycads presents many interesting features, but the account here 

 must be very brief. The pollen grain (microspore) is a rudi- 

 mentary male prothallium as in the pine, but some of the sterile 

 prothallial cells persist in the mature small spore (microspore). 

 After pollination the tube cell grows down into the tissue of the 

 nucellus (in Zamia) at one side of the endosperm forming a 

 haustorium which supplies the generative cell with food. The 

 pollen end of the tube then bends down into a cavity in the 



