SEEDS OF CYCADS PTERIDOSPERMS 



327 



D. 



female prothallus around forming a nutritive tissue, the endo- 

 sperm. The whole is enveloped by the thick integument, now 

 differentiated into three layers, which can even be recognised 

 in an immature form in the young ovule. These layers comprise 

 an inner (Fig. 186, C, ".), and a much thicker outer (o.), flesh, 

 with an inter- 

 vening very 

 hard stony 

 layer (s.). This 

 product of 

 megasporan- 

 gium, female 

 prothallus, and 

 embryo i s 

 known as a 

 seed, and is a 

 plum-like 

 structure many 

 times the size 

 of the original 

 ovule. 



The Cycads 

 obviously show 

 many super- 

 ficial resem- 

 blances to 

 Ferns, but 

 these are even 

 more pro- 

 nounced in 

 another group 



FIG. 187. A, Germinating microspore of Cycas, 

 showing vegetative cell (v.c.}, antheridial cell (a.c.}, 

 and tube cell (i.e.). B, Pollen tube of same with 

 the two spermatozoids (s.). C, Diagram of longi- 

 tudinal section through apex of nucellus (nu.) and 

 female prothallus (p.) of Dioon edule, showing 

 pollen tubes (p.t.) and pollen grains in various 

 stages of development, spermatozoids (s.), and 

 archegonia with eggs (o.) and necks (n.). D, Two 

 proembryos of Dioon edule, the left-hand one 

 younger than the right-hand one. e., embryo; 

 sp., suspensor. (A and B after Ikeno ; C and D 

 from Chamberlain.) 



of plants 



(Pteridosperms) which, like the Calamites and Lepidodendrons 

 of the Coal Measures, are -known only as fossils. The members 

 of this group, though closely resembling the Ferns in habit, 

 show analogies with Cycads in their anatomy, their mode 

 of reproduction, and especially in the possession of seeds. 



A complete knowledge of such fossil plants is only acquired 

 gradually and as a result of prolonged research. At first the 



