630 A TEXTBOOK OF THEORETICAL BOTANY 



The Prothallus 



The spores germinate immediately on reaching the ground, and divide 

 to form two cells of very unequal size. From the smaller cell there grows out 

 the first rhizoid, whilst the larger cell produces the prothallus. This pro- 

 thallus usually forms a flat plate of green tissue from which many filamentous 

 branches are given out (Fig. 639). The tissue is mostly one cell thick and 

 there is no apical growth. 



ANTHERIDIA 



ARCHEGONIA 



Fig. 639. — Equisetum arvense. Two prothalli. A, Young stage with 

 antheridia. B, Old stage with vegetative lobes and archegonia. 

 {After Sachs.) 



The prothalli are of two kinds, small male prothalli which only produce 

 antheridia, and large prothalli which are hermaphrodite and give rise to 

 archegonia and antheridia. The large prothallus is much more branched 

 than the small male and gives rise to a series of lobes on the lower side, 

 between which the archegonia are produced. In both types of prothalli, 

 as growth continues, numerous rhizoids are produced from the lower surface. 



Development of the Antheridium 



The antheridium is a very simple structure (Fig. 640). It arises from a 

 superficial cell of the prothallus, which divides into two by a wall parallel 



