178 MORPHOLOGY OF GYMNOSPERMS 



in a fossil. Such confusion of inner fleshy layer and nucellus in many 

 paleozoic seeds, however, in which the testa and nucellus are free, 

 seems impossible. In an ovule of this character, the inner set of 

 vascular strands could not make the very evident connections with 

 the region of the pollen chamber without traversing the nucellus. In 

 such cases, therefore, it is by no means inconceivable that the inner 

 fleshy layer of the testa was not represented. 



In whatever character the seeds of Cordaitales differ in structure 

 from modern seeds, these peculiarities are shared by Cycadofilicales ; 

 and this association is further emphasized by the entire absence of 

 any trace of an embryo in any of the sectioned paleozoic seeds (p. 46). 



3. The gametophytes 



Pollen grains of the Cordaitales have been found in the sporangia, 

 in the pollen chambers, and in the passageways ("necks") leading 

 to them (figs. 206, 209). The grains are comparatively large, and 

 with a rough outer coat; and they are said to be larger in the pollen 

 chambers than in the sporangia, indicating growth after pollination. 

 The striking feature, however, is the presence of a group of internal 

 cells (fig. 209), which are said to be more numerous after pollination 

 than before. Such a cell plexus has been observed in the pollen 

 grains of Cycadofilicales, and was discussed in that connection (p. 49). 

 It seems to be a safe inference that several, if not all, of these cells 

 were spermatogenous, and that swimming sperms were liberated 

 when the archegonial and pollen chambers had become continuous 

 by the breaking down of the intervening tissue, as in Ginkgo and the 

 cycads. Scott (18) has called attention to the interesting fact that 

 in 1896 Renault (6) suggested this possibility, before the announce- 

 ment of the discovery of swimming sperms in Cycas and Ginkgo. 

 No traces of pollen tubes have been observed in any of the paleozoic 

 seeds, although pollen grains have been found in the pollen chamber 

 after it had extended entirely through the nucellus to the megaspore 

 membrane, bringing the pollen grains into direct contact with the 

 female gametophyte. It is very probable that the sperms were dis- 

 charged from the pollen grains about as from the microspores of 

 modern heterosporous pteridophytes. 



In pollen grains found in the pollen chambers of ovules not defi- 



