[Chap. L THE SEED PLANTS 685 



about the time the megaspore is developing within the nucellus. Soon 

 after poHination, the scales of the ovulate cone close and the whole 

 cone becomes inverted. Sometimes the outer surface of the cone is prac- 

 ticallv sealed by exuded resin. 



There is little development of the male gametophyte during the rest 

 of the year, except the formation of a very short pollen tube from each 

 pollen grain. The tube does not grow through the nucellus until the 

 following spring, although the distance is extremely short. In the mean- 

 time the generative cell has divided into two cells, one of which divides 

 and forms the two sperms. 



During this first year the megaspore has undergone considerable de- 

 velopment, having formed a body of tissue often called "endosperm."' 

 This body of tissue is in reality the female gametophyte. At the end of 

 the gametophyte nearest the micropvle two or three archegonia develop 



Fig. 326. Diagram of a vertical section of a pine ovule and the scale to which 

 it is attached and of the male and female gametophvtes at the time of fertilization: 

 pr represents the prothallus with two archegonia; //) represents the integument, rut 

 the nucellus, m the micropyle, and p pollen tubes, two of which have reached 

 the neck cells of the archegonia. Redrawn from Strasburger. 



(Fig. 326). When mature, each archegonium consists of eight small 

 neck cells, a ventral-canal cell, and an egg. When the pollen tube grows 

 between the neck cells to the egg, it swells and bursts and the two 

 sperms are discharged. One spenn disintegrates and the other fuses 

 with the egg. 



The fertilized egg or zygote with its diploid chromosome comple- 

 ment is the beginning of a new sporophyte. Its development is de- 



^ Not to be confused with the real endosperm of flowering plants. The cells of the pine 

 "endosperm" are monoploid; tliose of the corn endosperm are triploid. 



