210 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



of Crotalaria sagittalis. Cook regarded these as examples of para- 

 sitization of the male gametophyte on the female. 



It has also been suggested that the pollen tube may sometimes 

 serve as a haustorial organ, not for its own benefit but for that of the 

 embryo sac or embryo. Longo (1903) believes this to be the case 

 in Cucurbita. He found that owing to a cutinization of the walls 

 of the nucellar epidermis and the formation of a suberized hypostase, 

 the embryo sac becomes cut off from the usual sources of its food 

 supply. The pollen tube is said to compensate for this deficiency. 

 As it approaches the embryo sac, it expands into a large swelling or 

 "bulla," which gives out a number of branches. One of these pene- 

 trates the embryo sac and effects fertilization, but the others ramify 

 into the tissues of the nucellus and inner integument, absorbing food 

 materials from the adjacent cells and transmitting them to the 

 embryo. 



A somewhat similar phenomenon has been reported in certain 

 members of the Onagraceae (Werner, 1914; Tackholm, 1915). The 

 pollen tube becomes greatly broadened in the micropyle 9 and often 

 sends out branches into the outer integument and the nucellus, while 

 the tip continues to grow toward the embryo sac, destroying the 

 cells lying in its path. The tube is recognizable even when the 

 embryo has attained a fairly large size, and it probably serves to 

 absorb food materials from the surrounding tissues and transmit 

 them to the embryo. 



A broad and massive pollen tube has also been seen in Carica 

 papaya (Foster, 1943). It persists for about eight weeks after 

 fertilization and probably helps in conveying food materials to the 

 embryo. In Ottelia alismoidcs, Hydrilla verticillata (Maheshwari 

 and Johri, 1950), and Oxybaphus nyctagineus (Cooper, 1949) the 

 pollen tube persists throughout the development of the seed. Ac- 

 cording to Cooper, it acts as a haustorium and transports nutritive 

 materials from the secretory cells of the funiculus to the embryo. 



9 A broad and persistent pollen tube may sometimes be mistaken for a synergid 

 haustorium or a micropylar extension of the embryo sac. Karsten (1891) reported 

 that in Sonneratia the fertilized embryo sac bored a hole through the wall layers 

 and came in direct contact with the nucellar epidermis, but Venkateswarlu (1937) 

 and Mauritzon (1939) have shown that this is incorrect and that Karsten was 

 actually looking at the pollen tube. 



