3 68 



GENERAL MORPHOLOGY OF PLANTS 



breaks through later, and curves upward in the sinus of the heart- 

 shaped prothallium, and takes on a green color as it comes to the 

 light. The stem grows more slowly. The embryo is now es- 

 tablished and the prothallium disappears, though it remains 



Embryo of fern (Adiantum concinnum) still surrounded by the archegonium, which has 

 grown in size, forming the "calyptra." L, leat; 5, stem; R, root; F, foot. 



attached to the young fern for some time. As the fern reaches 

 the age for spore production the life cycle is completed. 



522. Dimorphism in ferns. Some ferns have two kinds of 

 leaves, that is, leaves of different form, each kind performing a 

 different function for the plant. An interesting example is seen 

 in the stag-horn fern (Platycerium alcicorne) . This grows in the 

 tropics on tree trunks quite high up from the ground. It is 

 often grown in greenhouses in this climate. One kind of leaf is 

 narrow, and branched something after the fashion of the antlers 

 of a stag. These leaves are either " fertile" (spore-bearing) or 

 " sterile." Another kind of leaf is broad and hugs closely against 

 the base of the plant and the tree trunk. Here it catches falling 

 leaves which decay, hold water for the use of the fern, and the 



