15° 



MORPHOLOGY OF GYMNOSPERMS 



ary between the two becomes indistinct (fig. 177). Nothing is known 



of the behavior of chromatin during fertihzation. 



The prominent development of the pollen tube as an absorbing 



organ suggests a question as to its original 

 significance. In the Cycadofilicales the 

 pollen grains are in close proximity to the 

 egg, and there is nothing to indicate that 

 any pollen tube was formed. In the cycads 

 and in Ginkgo the pollen tube in the earlier 

 stages of its development functions only as 

 a haustorium. The pollen grain end, with 

 its cell complex, projects into the pollen 

 chamber and gradually approaches the egg, 

 not by actively invading any tissue, but 

 merely by elongating into the enlarging 

 pollen chamber. The portion which actively 

 invades the nucellus is somewhat longer than 

 the part in the pollen chamber, but has a 

 much smaller diameter and never carries 

 the sperms. In all other living seed plants, 

 the pollen grain remains just where it falls, 

 and the pollen tube acts both as a haus- 

 torium, actively invading the tissue, and as 

 a sperm carrier. It seems safe to assume 



that the pollen tube was originally a haustorium, and that its function 



as a sperm carrier was developed later. 



4. The embryo 



In 1877 Warming (8) described some of the later stages in the 

 development of the embryo of Ceratozamia, but in all cases the embryo 

 had already passed through the base of the egg and invaded the 

 endosperm. In 1884 Treub (13) gave a comparatively full account 

 of the embryogeny of Cycas circinalis, and this account was to some 

 extent confirmed and supplemented by Ikeno's (27) study of Cycas 

 revoluta. Later, Coulter and Chamberlain {2>2>) investigated 

 Zamia jioridana (figs. 178-181), and more recently Chamberlain (70) 

 has described the embryogeny of Dioon edule (figs. 183-185). 



Fig. 177. — Zamia Jiori- 

 dana: fertilization; the 

 smaller sperm nucleus has 

 pressed into the egg nu- 

 cleus, but still retains its 

 contour; the ciliated band 

 is still visible at the top 

 of the egg; X20. — After 

 Webber (26). 



