202 MORPHOLOGY 



carrying the sperms to the archegonia, but is simply an absorptive 

 organ. 



Embryo. — The development of the embryo differs in detail among the 

 cycads, but the general features are fairly constant (figs. 457, 4S8)- The 

 first stage is that of free nuclear division, resulting in a distribution of 

 nuclei through the cytoplasm of the huge egg (fig. 457). For example, 

 in Zamia there are eight successive divisions, re- 

 sulting in 256 free nuclei. The next stage is that 

 of wall formation, which varies in extent, sometimes 

 resulting in a tissue which nearly fills the egg, but 

 sometimes (as in Zamia) being restricted to the 

 lower region of the egg (fig. 458). This tissue, 

 which in Zamia occupies the lower end of the egg, 

 is the proembryo. The cells behind the tip cells of 

 the proembryo begin to elongate remarkably (fig. 

 459), forcing the tip cells out of the archegonium 

 into the nutritive tissue of the gametophyte {endo- 

 sperm), and continue to elongate until they form an 

 exceedingly long, tortuous, and often spirally coiled 

 massive filament (suspensor), sometimes 5 cm. or 

 more long (fig. 460). The tip cells at the end of 

 this long suspfensor form the embryo, which develops 

 two cotyledons, a constant feature of cycads. In 



the germination of the seed these cotyledons remain 

 Fig. 460. — Embryo ., • ,, . . 

 c ^ • ■ ;• withm the testa. 



of Cycas ctrcmalis, 



showing (above) the changes in terminology. — In passing from pteridophytes 



, . ^. ^, io spermatophytes, it is sometimes confusing to the beginner 



bryonic tissue, the .-,,,., , , , 



long and coiled sus- *° "' "^^ °'°^'' termmology of the seed plants to the more 

 pensor, and the termi- recent terminology of the lower groups. It is important that 

 nal embryo. — After this change in terms should not give rise to the idea that 

 Treub. there is any change in the character of the structures. The 



following list should prevent any possibility of confusion in 

 this transition from one set of terms to another. The real name of the structure, 

 as used in the lower groups, is followed in each case by the older name applied 

 in seed plants before any such relationship was known : microsporophyll (stamen), 

 megasporophyll (carpel), microsporangium (pollen sac), megasporangium (ovule), 

 microspore (pollen grain), megaspore (embryo sac), female gametophyte (endo- 

 sperm). 



Conclusions. — The fernlike characters of the cycads appear in the 

 vascular anatomy, mesarch and even concentric bundles still being pres- 



W 



