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A TEXTBOOK OF THEORETICAL BOTANY 



enclosed in a single layer of tissue. The peripheral cells now divide radially 

 so that a ring of five or six cells is formed surrounding the central one. All 

 the cells then divide transversely, differentiating the archegonium into two 

 halves, the lower half forming what is known as the venter, and the upper 

 forming the neck. 



The central cell of the venter now divides unequally, forming a small, 

 upper, ventral canal cell and a large lower cell, the oosphere. The 

 peripheral cells of the venter divide tangentially to form a wall two cells 

 thick. Meanwhile the neck elongates by the transverse division of its cells, 

 forming thereby a tubular wall, one cell thick, which encloses a long neck 

 canal, occupied at first by a row of thin-walled cells. Finally the cap cell 

 divides into four. 



The archegonia are produced in groups on the upper side of the thallus 

 just behind the growing point (Fig. 376). They stand on the vertical face 



Neck 



Oosphere 



Paraphyses 



Fig. 376. — Pellia epiphylla. Section through involucral 

 pocket showing archegonia covered by involucre. 



of a transverse ridge of tissue, facing towards the growing point of the thallus 

 and lying almost horizontally. A delicate membrane grows from the top of 

 the ridge, covering the archegonia as in a pocket. This cover is called the 

 involucre. 



In Pellia both male and female organs are borne on the same plant. When 

 the archegonium is ready for fertilization mucilage is formed by the dis- 

 organization of the neck canal cells. This mucilage absorbs water, swells, 

 and thus forces open the neck of the archegonium by separating the cap cells. 

 At the same time the cells of the antheridium take up water, swell, and 

 eventually burst the wall, liberating the antherozoids. These antherozoids 



