FERTILIZATION 183 



Pollen grains may also germinate on other parts of the flower be- 

 sides the stigma. In cleistogamous flowers (Frisendahl, 1927; 

 Madge, 1929; West, 1930; Maheshwari and Singh, 1934) germina- 

 tion takes place within the anther loculi (Fig. 107C), and in Aeginetia 

 indica (Juliano, 1935) even on the moist surface of the corolla tube. 

 Frequently pollen grains germinate on a foreign stigma, i.e., stigma 

 of a different species (see Eigsti, 1937; Sanz, 1945). If fertilization 

 takes place, it results in the formation of interspecific and inter- 

 generic hybrids. 



Course of Pollen Tube. After the tube has emerged from the 

 pollen grain, it makes its way between the stigmatic papillae into 

 the tissues of the style. The latter is extremely variable in length. 

 In some plants it is so short that the stigma is described as sessile, 

 while in Zea mays the so-called "silk" may attain a length of 50 cm. 



Depending on the presence or absence of the transmitting tissue 

 and on the extent of its development, styles have been classified into 

 three main types called open, half -closed, and closed (Hanf, 1935). 

 In the first type there is a wide stylar canal and the inner epidermis 

 itself assumes the function of the nutrition and conduction of the 

 pollen tube, as in the Papaveraceae, Aristolochiaceae, Ericaceae, 

 and many monocotyledons. In the second type the canal is sur- 

 rounded by a rudimentary transmitting tissue of two or three layers 

 of glandular cells, as in several members of the Cactaceae. In the 

 third or closed type, illustrated by Datura and Gossypium, there 

 is no open channel but instead a solid core of elongated and richly 

 protoplasmic cells through which the pollen tube grows downward 

 in order to reach the ovary. Finally, there are other plants like 

 Salix, Acacia, and many grasses in which the styles are solid but are 

 not provided with any specialized transmitting tissue. 



In open styles the pollen tube grows on the surface of the cells 

 lining the stylar canal (often in the mucilage secreted by them); 

 and in solid styles through the intercellular spaces between the cells 

 of the transmitting tissue, enlarging the spaces by the hydrostatic 

 pressure of its contents and secreting some enzymatic substances 

 which bring about a dissolution of the middle lamellae. Only rarely 

 does the pollen tube pass through the cells themselves. 



After arriving at the top of the ovary, the tube may enter the 

 ovule either through the micropyle or by some other route. The 

 former is the usual condition and is known as porogamy, but even 



