148 



GYMNOSPERMS 



Occasionally there are more than two cotyledons, three being re- 

 ported for Encephalarlos; and Ceratozafnia has regularly only one. 

 The cotyledon situation in Ccratozamia is particularly interesting. 

 Sister Helen Angela,'^^ noticing a few tracheids in the part of the 

 embryo opposite the cotyledon, suspected that they might represent 

 the missing cotyledon. Ccratozafttia is unique in 

 that the cone decays and sheds its seeds before 

 the cotyledon stage. She revolved seeds on a 

 clinostat during the entire embryogeny, and all 

 these seeds developed two cotyledons, while more 

 than ICO seeds, grown in the usual way, had only 

 one cotyledon, which developed on the side next 

 to the ground. In all cases, the cotyledons, in the 

 basal region, form a continuous cotyledonary 

 tube. At the tips, cotyledons are sometimes lobed 

 or divided, giving an impression that there are 

 more than two. 



Fig. 164. — Dioon 

 edule: mature seed: 

 The darkly shaded 

 part at the top is the 

 coleorhiza (c); the 

 two cotyledons and 

 first leaf are shown; 

 the dotted part is a 

 female gametophyte; 

 the hatched part is 

 the stony layer (s) of 

 the seed, outside of 

 which is the outer 

 fleshy layer (0); m, 

 micropyle; natural 

 size. From Cham- 

 berlain, The Living 

 Cycads'^" (University 

 of Chicago Press). 



THE SEEDLING 



There is no resting stage in a cycad seed : de- 

 velopment is continuous from fertilization to the 

 death of the old plant. Seeds do not long retain 

 their vitality. Those with the hardest stony layer 

 and toughest outer fleshy layer may be germi- 

 nated a year after they fall out from the cone. 

 Seeds of Macrozamia moorei, which have an ex- 

 ceptionally thick and hard stony layer, have ger- 

 minated after 2 years. If a seed rattles when shak- 

 en, the chances are against any germination. 

 Germinatio7i of the seed. — Cycad seeds are nearly always elongated. 

 When they fall out from the cone they do not sink into the ground, 

 but lie on the surface. To secure the best germination in the green- 

 house, they should not be covered with soil, but pressed in lightly — 

 about half-way — with the long axis of the seed parallel with the sur- 

 face of the soil. 



As the embryo within the germinating seed elongates, it fractures 

 the stony coat, the hard coleorhiza protecting the delicate young 



