12 THE COTTON PLANT IN EGYPT chap. 



discuss the metliods by which the pollen-grain reaches the 

 style, whether of its own, or of another flower, and also to 

 inquire into the pseudo-parasitic nature of the pollen tube, 

 which attacks some styles more easily than others. For the 

 present we will examine a normal ease. 



The sugar solution excreted by hairs on the style retains 

 the pollen-grain, and causes it to germinate. The single 

 pollen tube traverses the tissue of the style and the 

 conducting tissues till its end enters one of the loculi, 

 along the wall of which it passes till it finds (Fig. 3) 

 the micropyle of an ovule. Traces of branching may be 

 seen at this point. Passing through the micropyle 

 channel to the nucellus, it bores through the tissues of the 

 latter, and after literally squeezing its way through the 

 firmer wall of the megaspore, the end of the tube swells 

 up and bursts (Figs. 21, 22). From the torn end escape the 

 two male gametes, one of which passes to and fuses with 

 the egg-cell (Figs. 23, 24), forming a zygote, and thus 

 beginning a new life-history. The other male fuses with 

 the two polar nuclei, and the triple nucleus thus formed 

 (Fig. 25) serves later to provide the endosperm. 



The process is exceptionally rapid. Fertilisation is 

 normally completed within thirty hours after the first 

 opening of the flower, i.e. by the afternoon of the 

 following day. 



The embryo.— From the date on which the flower opens 

 until the boll cracks, some forty to sixty days later, 

 according to the weather and the kind of plant, the 

 embryo is developing inside the fertilised ovule, or seed. 



On the third day of this period the unicellular zygote 

 divides along a plane at right angles to the axis of the 

 ovule, its forty chromosomes, formed by the addition of 

 two twenties from each gamete, being each halved and 

 distributed to the daughter-cells (Fig. 26). Two more 

 divisions pi'oduce an octant (Fig. 27), and by the end of 

 the week the embryo is just visible to the naked eye, 



