178 MORPHOLOGY OF THE ANGIOSPERMS 



coniferophyte gymnosperms — Coniferales and Ephedrales — are so un- 

 like angiosperms as to be unquestionably a stock unrelated in origin to 

 the angiosperms, and their pollen tubes are probably of independent 

 origin. The cycads and Ginkgo have tubes functionally different from 

 those of conifers and angiosperms. In these taxa, the tubes, short and 

 haustorial in nature, develop on one side of the pollen grain. The pollen 

 grains become enlarged with increased cytoplasm and break open, dis- 

 charging the sperms and cytoplasm into a pollen chamber, where the 

 sperms swim in the cytoplasmic fluid to the archegonial necks. In the 

 angiosperms, nonswimming sperms are discharged into the embryo sac. 



Germination of pollen grains at a distance from the ovule has been 

 considered an angiosperm character, but, in Tsiiga and Araucaria, the 

 grains germinate far from the ovule and form long tubes extending to 

 the micropyle; the pollen grains of Agathis form extensive pollen tubes 

 in the cone axis and ovular tissues before reaching the archegonium. 

 Stages in the phylogenetic development of germination at a distance 

 from the ovules are present in living Ranales. In Dcgeneria and some 

 species of Drhmjs, the pollen grains germinate on a long, ventral stig- 

 niatic crest, and their short tubes penetrate between the unsealed "lips" 

 of the carpel directly to ovules close by (Fig. 83). In other species of 

 Drimijs and other genera of the Winteraceae, progressive acropetal 

 shortening of the crest — the pollen tubes still short — restricts ovule 

 position to the area directly below the crest. With the acquisition of a 

 terminal stigma and a style, the pollen tubes become long, and ovule 

 position is not restricted. 



The pollen tube, as it increases in length, usually pushes between 

 the cells of the stigmatic surface and the inner tissues of the stigma 

 and style. In some families, it destroys the adjacent cells, eroding a 

 path to the ovules. Its course may be along the surface of the stylar 

 canal or deep within the stylar tissues. It may pass through vascular 

 tissues (protoxylem) in its course — Casiiarino. (It resembles in this the 

 course of pollen tubes in the phloem of araucarian conifers. ) Where a 

 stylar canal is present, the tube may pass wholly or partly along the 

 wall of the canal. Entering the ovarian chamber, the tube may continue 

 along the wall to the ovule or cross directly to the micropyle. It follows 

 lines of transmitting tissue (often wrongly called conducting tissue), 

 the extent and distribution of which within the stylar canal and the 

 ovary have not commonly been recognized. The course of the tube, 

 though obviously toward the ovules, has been considered not directed 

 structurally. But, in many taxa, the tube follows direct lines through 

 the ovary, predetermined by the distribution of transmitting tissue (Fig. 

 77), which is by no means restricted to the style, as often implied. In 

 the more specialized taxa, the tube seems to take the shortest course 



