DEVELOPMENT OF THE PliOTHALLIUM. 



135 



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

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

 whieli speedily convert this portion into a broad flat plate at 

 first only one cell thick, but eventually several cells tliick al(jng 

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

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

 (Fig. 69), but by further grow^th anteriorly, by an apical cell or 

 otherwise, the wider end becomes 

 still more ilattened 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 proton ema 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- 



FiG. 69, (After Suminski.)— Older pro- 

 thallium, showing two rhizoids, three 

 young antheridia, and numerous 

 chlorophyll-bodies. 



