DEVELOPMENT OF PLANTS 



247 



surface are also found on the under side of the thallus (Fig-. 

 180, A). These plates curve up around the growing point and 

 doubtless protect it against drought, in which work they are as- 

 sisted by the mucilage cells or glands. The development of these 

 glandular cells on the plates, as well as on other parts of the 

 thallus, adapt the plants to the drier terrestrial conditions to which 

 they are exposed, as was the case with the mucilaginous walls of 

 the Schizomycetes and Zygnematales. 



(b) Sexual Reproduction of Ricciocarpus. — The gametes are 

 produced in more complex gametangia than we have as yet seen. 

 These organs are developed upon the upper surface of the thallus 

 and in some species appear as lines radiating from the center of 

 the plant. The male gametangium originates from one of the 

 superficial cells of the thallus, which at first continues to divide 

 transversely after the manner of the vertical plates of chlorophyll- 

 bearing cells (Fig. 181, A). The cells of these vertical plates 



Fig. 181. The origin and structure of the male gametangia or an- 

 theridia : A, section of the thallus, showing the apical cell, x, and the early 

 stages, a, b, in the development of the antheridia. B, older antheridium 

 with cells dividing vertically. C, later stage in which the wall cells are 

 differentiated. D, mature antheridium of Marchantia, showing the nu- 

 merous cells that develop the male gametes and the wall cells, w. E, 

 greatly enlarged view of a male gamete after discharge from the an- 

 theridium. 



