SPERMATOPHYTA 



323 



wall or from the sporogenous tissue. The number of spores produced by- 

 each sporangium is high, ranging from 500 in Zamia to 30,000 in Dioon. 

 Dehiscence occurs by means of a longitudinal slit. In general, the micro- 

 sporangium of the cycads shows a striking resemblance to the sporangium 

 of the Marattiales, particularly to that of Angiopteris (Fig. 229). 



Ovulate Strobilus. In most genera the ovulate cones are borne singly, 

 but in Macrozamia and Encephalartos they may occur in groups of two, 

 three, or more. They are com- 

 posed of many spirally arranged, 

 fleshy megasporophylls (carpels). 

 In Cycas the megasporophylls are 

 very loosely arranged to form a 

 rosette that surrounds the stem tip, 

 which later continues its growth 

 upw^ard through the rosette (Fig. 

 270A). In Dioon the megasporo- 

 phylls form a loose cone, but in all 

 the other genera they are compactly 

 organized (Figs. 267 and 270C). 

 The megasporophylls of cycads 

 exhibit a striking reduction series, 

 ranging from pinnate types with 

 six or eight ovules, in most species 

 of Cycas, to peltate types wdth 

 only two ovules, in the other genera 

 (Fig. 273). Throughout this series 

 the megasporophylls become less 

 and less leaf-like and the strobilus increasingly more compact. 



The main body of the ovule is the nucellus or megasporangium proper. 

 It is surrounded by a single massive integument free from the nucellus only 

 at its upper end and forming a narrow passageway, the micropyle (Fig. 

 274). A prominent nucellar beak is developed, in the center of which a 

 pollen chamber later arises. The integument consists of an outer fleshy, 

 a middle stony, and an inner fleshy layer. Vascular strands are found in 

 both fleshy layers. They are composed of mesarch xylem. Deep within 

 the nucellar tissue a megaspore mother cell becomes differentiated. It 

 gives rise to a linear tetrad of megaspores. Of these, only the innermost 

 megaspore functions, the other three degenerating. 



Female Gametophyte. As in all seed plants, the megaspore germi- 

 nates in situ, producing the female gametophyte. Its formation involves 

 several stages. First, the megaspore enlarges and free-nuclear division 

 occurs. Then, by further enlargement, a central vacuole is formed, 

 resulting in a parietal placing of the nuclei. As nuclear division proceeds, 



Fig. 271. Microsporophylls of Cycas cir- 

 cinalis, showing back (abaxialj and side 

 views, one and one-half times natural size. 



