720 



A TEXTBOOK OF THEORETICAL BOTANY 



Protoxylei 



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secondary xy/em,^,__f^ '1^^^^^^ ^; 

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Fig. 724. — Cycas revoluta. Transverse section of a 

 young prirriary root showing diarch primary 

 xylem separated from the early secondary xylem 

 by parenchyma. 



Reproduction 



The cones of Cycas are terminal on the main stem, the sporophylls being 

 formed spirally in succession with the foliage leaves. In the female strobilus 

 the vegetative apex is not affected and continues its growth directly through 

 the cone, but in the male an axillary bud is formed at the base of the cone which 

 displaces it laterally and becomes the new stem apex. This is also the usual 

 case in other Cycads. 



The vascular supply to the male cone takes the form of a ring of bundles 

 arising from the inner edge of the main vascular cylinder and converging 

 like a dome towards the base of the peduncle. Successive peduncular domes 

 may be seen in the pith in longitudinal sections of old stems. 





The Male Cone 



The microsporophylls are arranged in a close spiral on the male cone, 

 which may be 50 cm. long (Fig. 725). They are woody in texture, wedge- 

 shaped and blunt-ended. The lower surface of each is continuously covered 

 with microsporangia (pollen sacs) (Fig. 726), but closer inspection shows 

 that they are really grouped into sori of from three to six sporangia, arising 

 from a central papilla, to which the dehiscence slits are radial. The analogy 

 with Marattia is here fairly obvious, and indeed the grouped, or alternatively, 

 synangial arrangement of the microsporangia is a common feature which 



