160 



PTERIDOPHYTES 



(Fig. 100.) These dots contain niunerous sporangia (singular 

 sporangium) (Fig. 101), which are filled with spores. Each 

 sporangium is supported by a stalk and from a side view appears 

 to be flat, but as a matter of fact is slightly thicker in the centre 

 than on the margin. The marginal cells are highly specialized 

 and are of two kinds ; the thick-walled cells from about three- 

 fourths of the distance and the thin-walled cells for the re- 

 mainder of the distance. When the spores (Fig. 102) are 

 mature, the drying out of the sporangia results in an uneven 



Fig. 100. 



Fig. 101. 



Fig. 102. 



Fig. 98. — Young fern leaf showing method of unrolling. V 



Fio. 99. — Part of fern leaf showing sori or fruit clusters. 



Fig. 100. — Part of fern leaf showing sori with indusium. 



Fig. 101. — Sporangium from fern sorus. 



Fig. 102. — Fern spores from sporangium. 



tesnsion of the two kinds of cells, a bursting of the sporangium 

 and a scattering of the spores. 



Two Steps in Reproduction. — The spores germinate and 

 eventually form what is known as ihe protlialUum. (Fig. 103, e.) 

 In most ferns this prothallium is very small and somewhat 

 heartshaped. It is composed of chlorophyll bearing paren- 

 chyma cells and has many rhizoids which are very similar to the 

 root-hairs of the higher plants. They penetrate the moist soil 

 from which they derive both water and food in the same man- 

 ner as root-hairs. On the under surface will be found numerous 

 small bodies, the archegonia (singular archegonium) and the 



