188 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



Grevillea (Brough, 1933), and Cynomorium (Steindl, 1945), they 

 become mucilaginous or glandular and seem to contribute to the 

 nutrition of the pollen tube. In Cardiospermum (Kadry, 1946) not 

 only the cells belonging to the inner integument but also those 

 forming the apical portion of the nucellus give rise to a mucilaginous 

 mass which facilitates the entry of the pollen tube. In plants with 

 a many-layered nucellar tissue, like Beta (Artschwager and Starrett, 

 1933), those of its cells which are in continuity with the micropyle 

 become elongated and richly protoplasmic and give an impression 

 as though they were designed to lead the pollen tube through the 

 path of least resistance. 



It is of interest to note that even during its passage through the 

 nucellus the pollen tube usually makes its way between the cells 

 and not through them. Normally it causes but little disturbance 

 in their position and they soon return to their original shape, but in 

 a few families like the Lythraceae, Sonneratiaceae, Onagraceae, 

 and Cucurbitaceae the tubes are so broad that they destroy the 

 cells which lie in their way and cause a permanent break in the 



tissues. 



Entry of Pollen Tube into Embryo Sac. After penetrating the 

 wall of the embryo sac, the pollen tube may either pass between 

 the egg and one synergid as in Fagopyrum (Mahony, 1935), or be- 

 tween the embryo sac wall and a synergid as in Cardiospermum 

 (Kadry, 1946), or directly into a synergid as in Oxalis (Krupko, 

 1944), Elodea (Ernst-Schwarzenbach, 1945), and Daphne (Ven- 

 kateswarlu, 1947). In Viola it not only enters a synergid but is 

 said to force its way through the base of the latter (Madge, 1929). 



As a rule only one synergid is destroyed by the impact of the 

 pollen tube and the other remains intact until some time afterward, 

 but in Mimusops, Achras, and Bassia (Murthy, 1941) both are 

 destroyed and in Phryma (Cooper, 1941) and Tropaeolum (Walker, 

 1947) neither of them seems to be affected. 



In some genera, such as Tacca, Wormia (Paetow, 1931), and 

 Nelumbo (Ohga, 1937), the synergids degenerate even before the 

 entry of the pollen tube, and in others like Plumbago, Vogelia, and 

 Plumbagella (see Maheshwari, 1948) they are not formed at all. 

 This seems to indicate that they are not essential for fertilization, 

 and the view that they secrete substances which exercise a chemo- 

 tactic influence over the pollen tube, or that they act as shock 



