HAUSTORIAL ROLE OF POLLEN TUBES

In angiosperms, the pollen tube is siphonogamous and its main function is to carry the male gametes for double fertilization. In some taxa, as in Cucurbitaceae, the tube branches after entering the ovule, prior to fertilization. The tube may even swell and form a bulla. During post-fertilization development of the ovule, a portion of the tube may persist in the micropyle, or in the embryo sac, or in both, sometimes even in the micropyle of the mature seed. Haustorial function has been presumed in a number of taxa. In Grevillea, following fertilization, the pollen tube branches at the micropyle, and the branches grow intercellularly into the ovarian tissue where further branching occurs. A haustorial role of the pollen tube is presumed from circumstantial evidence. In gymnosperms (for example, Cycas, Zamia and Gmkgo) the pollen tube is non-siphonogamous, arises from the distal (upper) pole of pollen grain, and grows laterally in the apical region of the nucellus. The tube branches in Cycas and Ginkgo but remains unbranched in Zamia. These pollen tube branches are enucleate, and are not concerned with the transport of male gametes for fertilization. However, the haustorial role has been well documented. In Podocarpus, the pollen tube is siphonogamous and arises from the proximal (lower) pole of pollen grain. After traversing the nucellus, the tube forms a bulla at the point of contact with the female gametophyte, and several branches originate from the bulla. The pollen tube branches grow along the inner surface of the nucellus and the outer surface of the female gametophyte. The haustorial role of the pollen tube branches is uncertain. Procedures for convincingly demonstrating the haustorial role of pollen tubes are discussed. © 1992 Annals of Botany Company.