94 



THE BIOLOGY OF A PLANT. 



but by further growth in front, by an apical cell or otherwise, 

 the wider end becomes still more flattened and heart-shaped or 

 even kidney-shaped. Numerous rhizoids (so-called because they 



are not morphologically true roots) 

 are put down, and the whole struc- 

 ture assumes approximately the 

 appearance indicated in Fig. 58. 

 The spore-membranes and pro- 

 tonema soon fall away and the 

 prothallium enters upon an inde- 

 pendent existence, being rooted by 

 its rhizoids and having an abun- 

 dance of chlorophyll. In the broad 

 thin plate of tissue no subdivision 

 into stem and leaf exists, and the 

 plant-body closely resembles the 

 "thallus" of one of the lowest 

 plants. Since it is the precursor 

 of the ordinary " fern," it is called 

 the " prothallus" or " prothal- 

 lium" '" 



The cushion forms a promi- 

 nence on the lower side ; upon its 

 Posterior part most of the rhizoids 



borne. 



Sexual Organs of the Prothallium. 

 The prothallia of ferns are as a rule bisexual or double-sexed ; 

 that is, each individual possesses both male and female organs. 

 But the latter appear somewhat later than the former, and poorly 

 nourished prothallia often bear only male organs, though they 

 will frequently develop female organs also if placed in better cir- 

 cumstances. 



The Antheridia, or male organs, first recognized in ferns by 

 Niigeli in 18-44, are hemispherical prominences occurring upon the 

 posterior part and the under side of the prothallium, often among 

 the rhizoids. When fully formed (Fig. 58, Fig. 59) an anther- 

 idium consists of a mass of rounded cells (spermaiozoid mother- 

 cells] enveloped by a membrane one cell in thickness. 



antheridia and numerous chlorophyll- 

 bodies. 



* The protluillium of ferns was discovered by Ehrhart in 1788. 



