j8 A TEXTBOOK OF THEORETICAL BOTANY 



They arise as small colourless outgrowths which become cut off from the 

 parent cell. These antheridia are flask-shaped cells, each of which gives rise 

 to a single, colourless, oval antherozoid, provided with a pair of apical 

 flagella. The antherozoids are set free by the breakdown of the apex of the 

 wall of the antheridium. 



The oogonium is formed terminally on a short lateral branch from a 

 vertical filament, but it is usually pushed into a lateral position by the up- 

 growth of a branch arising from the underlying cell. The oogonium is flask- 

 shaped. The basal part is swollen and contains the chloroplast, while the 

 neck is prolonged into a tube or trichogyne, which contains only colourless 

 cytoplasm. When mature the tip of the trichogyne breaks down and some 

 of the cytoplasm is extruded, while the basal part of the oogonium rounds 

 off and forms a single oosphere. 



Fertilization takes place by the antherozoid entering the trichogyne and 

 passing down to the oosphere. The nucleus of the male gamete is con- 

 siderably smaller than that of the female, but as the male gamete migrates 

 towards the female nucleus, its nucleus increases in volume so that at the time 

 of actual fusion the two nuclei are of approximately equal size. 



After fertilization the oogonium enlarges, while the neck is cut off by a 

 septum and a wall is laid down around the oospore so formed. This wall 

 is thick and brown in colour and may be partly derived from the wall of the 

 oogonium. Meanwhile new branches arise from the underlying cells of the 

 branch bearing the oogonium, and grow around and envelop the oospore 

 forming a pseudo-parenchymatous investment. These cells subsequently 

 die and the oospore is shed enclosed both in its own wall and in that derived 

 from the pseudo-parenchyma, which becomes fused to it. In this state it 

 remains over the winter. 



The following spring the contents gradually assume a bright green colour 

 and divide (Fig. 60), first into two by a wall perpendicular to the long axis, 



Fig. 60. — Coleochaete scutata. Sections through two 

 germinating oospores showing division of the contents. 

 The left-hand oospore shows stages of meiosis. 



