THE MALE GAMETOPHYTE 179 



along the surface of the ovary wall and placenta to the ovule. To reach 

 the micropyle, the tube may pass along the surface of the ovule to the 

 micropyle, but it may enter the ovule and follow the vascular bundle, 

 running through protoxylem cells in funicle and raphe — Casuarina. The 

 crossing of a space, as a "short cut" to the micropyle — described in 

 some taxa — has been called an "extraordinary accomplishment," be- 

 cause it involves the continuing growth of the tube tip, without contact 

 of the tip with tissue of the ovary. Where no ovule, as such, is present 

 — some parasites, such as Viscum — the tubes pass through carpellary 

 tissues. Branching of the tube is rare except in chalazogamous taxa, 

 where it may take place at various levels in the ovule. 



Time of growth of tube from stigma to embryo sac varies greatly and 

 is not controlled by distance from stigma to embryo sac; the tip may 

 grow rapidly or slowly, and many factors enter into its elongation. The 

 tube may grow from pollen grain to the egg over some millimeters in 

 a few hours. But many days, even weeks and months, may pass between 

 pollination and fertilization — in the witch hazel, about six months; in 

 the black oaks, about twelve months; but these periods include winter 

 dormancy. 



Entrance of the pollen-tube tip directly through tissues of the ovule, 

 rather than by the micropyle, was first found in Casuarina in 1891 and 

 termed chalazogamij ( Fig. 103 ) , because the tube penetrates the chalaza 

 on the way to the embryo sac. Chalazogamy was soon found in several 

 other amentiferous genera, and, since the Amentiferae were then con- 

 sidered the primitive angiosperms, chalazogamy was considered a primi- 

 tive character. (A classification of angiosperms as Chalazogams and 

 Porogams was supported by a few botanists. ) But chalazogamy is now 

 known to be present in taxa in other parts of the angiosperm system — 

 Anacardiaceae, Cucurbitaceae, Rosaceae, Chloranthaceae — and must be 

 considered a specialized, rather than a primitive, character. Penetration 

 of the tube through the integuments is sometimes termed mesogamtj, 

 and the term hasigamij was suggested to replace chalazogamy. "True 

 chalazogamy" is said to occur only when the tube enters the embryo sac 

 at its base. Entrance of the tube by the micropyle has been called 

 porogamij; all other methods, aporogamij. In a few taxa, there is varia- 

 tion in place of entry of the tube — Epilobium, Brassica. Entrance of the 

 tube into the embryo sac at any point other than the micropylar end is 

 probably rare; tubes entering the ovule below the micropyle usually 

 pass along the embryo-sac wall until they reach the position of the egg. 

 Pollen tubes may persist in the nucellus long after fertilization, a con- 

 dition frequent in the Proteaceae. 



The parasitic behavior of the pollen tube is reported to reach an 

 extreme in Casuarina, where it is described as destroying germinating 



