FERNS. i6i, 



formation of a cross wall, and as it increases m length other 

 cross walls are formed. But very earl)- in its growth we see that 

 a slender outgrowth takes place from the cell nearest the old 

 spore wall. This slender thread 

 is colorless, and is not divided 

 into cells. It is the first rhizoid, 

 and serves both as an organ of 

 attachment for the thread, and for 

 taking up nutriment. 



554. Prothallium. — Very soon, 

 if the sowing has not been so 

 crowded as to prevent the young 

 plants from obtaining nutriment 

 sufficient, we will see that the end 

 of this protonema is broadening, 

 as shown in fig. 303. This is done 

 by the formation of the cell walls 

 in different directions. It now 

 continues to grow in this way, the 

 end becoming broader and broader, 

 and new rhizoids are formed from 

 the under surface of the cells. The 

 growing point remains at the mid- 

 dle of the advancing margin, and 

 the cells which are cut off from 

 either side, as they become old, 



widen out. In this way the y^^^^ pro.l,amur;^'f a f=rn (nipho- 



" wings," or margins of the t""'"^'- 



little, green, flattened body, are in advance of the growing 



point, and the object is more or less heart-shaped, as shown 



in fig. 297. Thus we see how the prothallium of ferns is 



formed. 



555. Sexual organs of ferns. — If we take one of the prothal- 

 lia of ferns which have grown from the sowings of fern spores, 

 or one of those which may be often found growing on the soil 



