DEVELOPMENT OF THE PROTHALLIUM. 



135 



growing protonema to serve as anchors and roots. At the oppo- 



site or distal end longitudinal partitions soon appear (Fig. 68), 



which speedily convert this portion into a broad flat plate at 



first only one cell thick, but eventually several cells thick along 



the median line. This thickening is the so-called ' ' cushion ' ' 



(see Fig. 70). The whole prothallium is now somewhat spatulate 



(Fig. 69), but by further growth anteriorly, by an apical cell or 



otherwise, the wider end becomes 



still more flattened and heart- 



shaped or even kidney-shaped. 



Numerous rhizoids (so-called be- 



cause they are not morphologi- 



cally true roots) are put down, 



and the whole structure assumes 



approximately the appearance in- 



dicated in Fig. 70. The spore- 



membranes and protonema soon 



fall away, and the prothallium 



enters upon an independent exist- 



ence, being rooted by its rhizoids 



and having an abundance 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 '"'"pro- 

 thallium" 



The cushion forms a prominence on the lower side ; upon 

 its posterior part most of the rhizoids are borne. 



Sexual Organs of the Prothallium. The prothallia of ferns 

 are as a rule bisexual or hermaphrodite ; 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 circumstances. 



The Antheridia^ or male organs, are hemispherical promi- 



young antheridia, and numerous 

 chlorophyll-bodies. 



