SPERMATOPHYTES 



183 



Fig. 421.- 



- Seeds of Aneimites (an American 

 form). — After White. 



lamina had disappeared and only the prominent ribs persisted. In 

 some cases, however, the seeds replace sori on ordinary fernlike leaves 

 (fig. 422). There are very many detached paleozoic seeds which have 

 never been connected with the plants that produced them ; but doubt- 

 less many of them belonged to the Cycadofilicales. So far as these 

 attached and detached seeds 

 have been sectioned, they show 

 certain features in common 

 which are regarded as primi- 

 tive. In seed plants the mega- 

 sporangium has long been 

 called an ovtde. In general 

 structure it consists of a central 

 region (the real sporangium) 

 called the nucellus, which is invested by one or two coats called integu- 

 ments. A passageway (micropyle) is left through the integument at 

 the tip of the nucellus. When the changes occur that transform the 

 ovule into the seed, the integument develops in various ways to form 

 the seed coat or testa. In fossil seeds it is evident that the structure 

 of the ovule must be inferred from the structure of the seed. 



In the seeds of Cycadofilicales 

 there is a three-layered testa, which 

 is often peculiarly free from the 

 nucellus. The vascular strand that 

 enters the seed divides into two sets 

 of branches, one set traversing the 

 testa, and the other traversing the 

 outer region of the nucellus, in case 

 the testa and nucellus are free. The 

 nucellus is beaked, and contains a 

 deep chamber (pollen chamber), 

 which serves as a gathering place 

 for microspores, and which in living gymnosperms is associated with 

 swimming sperms. A remarkable feature of the seed, and of all paleozoic 

 seeds that have been sectioned, is that there is no trace of an embryo. 

 Since the embryo is present in mesozoic seeds, its absence from paleozoic 

 seeds must be due to other causes than failure to be preserved. 



Stamens. — The microsporangiate structures (stamens), first recog- 

 nized in 1905, have been found to be of at least three types. They are 



Fig. 422. — Seeds of 

 ordinary fernlike leaves.— 



ElTRY. 



Pecopteris on 

 ■ After Grand' 



