328 



PLANT MORPHOLOGY 



Fig. 276. Development of the male gametophyte of Dioon edule. A, microspore with 

 nucleus in early prophase of first mitosis; B, shedding condition of pollen grain; C, beginning 

 of pollen tube formation, showing prothallial cell {p), generative cell {g), and tube nucleus 

 it); D, formation of stalk cell (s) and body cell (6) from generative cell; E, later stage, the 

 body cell much elongated and with two blepharoplasts showing conspicuous radiations; 

 A, B, C, X 1,235; D, X980; E, X618. (After Chamberlain.) 



The cycads are unique in that the seed germinates promptly, without 

 going into a resting stage. The stony coat is broken by the coleorhiza 



enclosing the elongating root tip. The 

 coleorhiza is soon destroyed by the root 

 tip, which then rapidly grows down- 

 ward into the soil. The stem remains 

 inconspicuous, but a leaf soon develops. 

 The cotyledons remain inside the seed 

 coat, absorbing food from the "endo- 

 sperm" and transferring it to the devel- 

 oping seedling. 



Summary. The cycads resemble the 

 ferns in their general habit, vascular 

 anatomy, form and venation of the leaves, occurrence on the microspo- 

 rophylls of abaxial sporangia in sori, structure of the microsporangia, and 

 multiciliate sperms. All these characters, as well as the ovule structure, 



B 



Fig. 277. Side (.4) and top (B) 

 views of a sperm of Zamia floridana, 

 showing numerous cilia on a spiral 

 band, X 100. (After Webber.) 



