478 FUNDAMENTALS OF FRUIT PRODUCTION 



size more rapidly than the filament and gives rise to a structm^e that is 

 generally grooved longitudinally on the outside and four-loculed in 

 cross section. Figures 1 to 8 of Plate II show successive stages in the 

 development of the male reproductive cell, or pollen grain, from the 

 tissues of the anther in the grape. At a comparatively early stage there 

 is a differentiation between the cells of its outer layers and those in the 

 interior. This differentiation has progressed rather far in the section 

 shown in Fig. 1, Plate II, the epidermal, middle, tapetal and mother- 

 cell layers being clearly distinguishable. Eventually the epidermal 

 and sub-epidermal layers undergo a series of changes which lead to 

 their separation from the sporogenous tissue within and to their assuming 

 the role of a simple protective shell or covering. Some idea of these 

 changes is afforded by Figs. 2 and 3 of Plate II. Figure 4 of Plate II 

 shows a single large pollen-mother-cell just previous to the reduction 

 division, which gives rise to four daughter cells, each of which is sur- 

 rounded by a membrane or cell wall. This is the so-called tetrad stage, 

 shown in Fig. 5, Plate II, though only three of the four microspores are 

 shown in the plane in which that figure was drawn. Shortly after the 

 formation of these tetrads the mother-cell wall breaks down and liberates 

 the microspores. Figure 6 of Plate II shows one of the microspores of the 

 Brighton grape just previous to its liberation and Fig. 7 of Plate II shows 

 one of the Concord variety a short time after its liberation. Its thick 

 wall, large nucleus and vacuole are prominent. Usually some time 

 before, though sometimes after, the dehiscence of the anther and the 

 dispersal of the pollen there are further changes within the pollen grain. 

 The nucleus divides giving rise to two daughter nuclei. One is called the 

 generative nucleus, because it alone gives rise to the gametes. This 

 generative nucleus becomes surrounded by a cell wall and is then called 

 the generative cell of the pollen grain. The other is called the vegetative 

 nucleus, because its function is more closely associated with germination 

 and because it functions as the nucleus of the pollen tube. Figures 9 

 and 12 of Plate II show two stages in the development of these two nuclei, 

 though both cases are somewhat abnormal because they show the initial 

 stages of a degeneration that leads to impotency. Figure 14 of Plate II 

 shows the generative cell and vegetative nucleus of a mature pollen grain 

 of the Concord grape before dehiscence. 



Pollination. — In the ordinary course of events the maturing of the 

 ovules and of the pollen grains is followed by a transfer of pollen from 

 stamen to stigma. If the transfer is from stamen to stigma of the same 

 flower or to the stigma of another flower on the same plant, or, in the 

 case of pomological varieties, to the stigma of a flower on any plant of 

 the same variety, the process is self pollination. If the transfer is to 

 the flower of another individual, or, in the case of pomological varieties, 

 to the flower of another variety, the process in cross pollination. When 



