274 PHYLUIM XII. CYCADOPHYTA 



watery fluid to the archegone, finally reaching the egg. 



From the fertilized egg there is later developed a little 

 sporophyte which is nourished for a 

 time by the tissue of the surrounding 

 gametophyte. In the meantime the 

 integument of the sporangium has 

 greatly thickened into a mass of tissue 

 Fio. i49.-zanna ^^^^ extcmally and stony internally. 



to°h"'tcr'' """^ ^^"'''' ^^'^^^^'^ ^^^ growth ceases the megaspor- 

 angium (ovule) with its contained 



gametophyte and sporophyte falls off, as the ''seed." 



491. After the fall of the seed when placed in proper 

 conditions as to moisture and temperature, the sporo- 

 phyte resumes its growth at the expense of the game- 

 tophyte (now called ''endosperm"), and soon sends out a 

 root, and later a green leaf, after which it becomes an 

 independent long-lived plant. 



492. The other living Cycads are essentiall}^ similar 

 in structure to the foregoing. All of the species are 

 tropical or subtropical. jVIany that 



lived in Mesozoic times have long 

 been extinct. 



493. In the Mesozoic period there 

 flourished a group of Cycads that may 

 be called the "Flowering Plant An- 

 cestors" (Class Bennettitineae), ^''^- ^?Pfl~^?M°^"^^'' 

 and which had "flowers" containing 



a central cluster of stalked megasporangia, surrounded 

 by a whorl of pinnate microsporophylls. Below these 

 were many sterile bracts reminding one of flower-leaves 

 (perianth). The resemblance of this primitive flower 

 to the flowers of the simpler Flowering Plants such as 

 Magnolia, Asimina, Ranunculus, etc., is so great as to 

 suggest a genetic relationship. 



